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30.2D: Stem Modifications - Biology

30.2D:  Stem Modifications - Biology


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Stem modifications, either aboveground, underground, or aerial, enable plants to survive in particular habitats and environments.

Learning Objectives

  • Explain the reasons for stem modifications

Key Points

  • Modified stems that grow horizontally underground are either rhizomes, from which vertical shoots grow, or fleshier, food-storing corms.
  • New plants can arise from the nodes of stolons and runners (an aboveground stolon): stems that run parallel to the ground, or just below the surface.
  • Potatoes are examples of tubers: the swollen ends of stolons that may store starch.
  • The stem modification that has enlarged fleshy leaves emerging from the stem or surrounding the base of the stem is called a bulb; it is also used to store food.
  • Aerial modifications of stems include tendrils, thorns, bulbils, and cladodes..

Key Terms

  • stolon: a shoot that grows along the ground and produces roots at its nodes; a runner
  • tuber: a fleshy, thickened, underground stem of a plant, usually containing stored starch, as for example a potato or arrowroot
  • cladode: green branches of limited growth which have taken up the functions of photosynthesis
  • rhizome: a horizontal underground stem of some plants that sends out roots and shoots from its nodes
  • corm: a short, vertical, swollen underground stem of a plant that serves as a storage organ to enable the plant to survive winter or other adverse conditions such as drought
  • bulb: the bulb-shaped root portion of a plant such as a tulip, from which the rest of the plant may be regrown
  • tendril: a thin, spirally-coiling stem that attaches a plant to its support
  • thorn: a sharp, protective spine of a plant
  • bulbil: a bulb-shaped bud in the place of a flower or in a leaf axil

Stem Modifications

Some plant species have modified stems that are especially suited to a particular habitat and environment. A rhizome is a modified stem that grows horizontally underground; it has nodes and internodes. Vertical shoots may arise from the buds on the rhizome of some plants, such as ginger and ferns. Corms are similar to rhizomes, except they are more rounded and fleshy (such as in gladiolus). Corms contain stored food that enables some plants to survive the winter. Stolons are stems that run almost parallel to the ground, or just below the surface, and can give rise to new plants at the nodes. Runners are a type of stolon that runs above the ground and produces new clone plants at nodes at varying intervals: strawberries are an example. Tubers are modified stems that may store starch, as seen in the potato. Tubers arise as swollen ends of stolons, and contain many adventitious or unusual buds (familiar to us as the “eyes” on potatoes). A bulb, which functions as an underground storage unit, is a modification of a stem that has the appearance of enlarged fleshy leaves emerging from the stem or surrounding the base of the stem, as seen in the iris.

Modifications to the aerial stems, vegetative buds, and floral buds of plants perform functions such as climbing, protection, and synthesis of food vegetative propagation. Aerial modifications of stems include the following:

  • Tendrils are slender, twining strands that enable a plant (like the buckwheat vine) to seek support by climbing on other surfaces. These may develop from either the axillary bud or the terminal bud of the stem.
  • Thorns are modified branches appearing as hard, woody, sharp outgrowths that protect the plant; common examples include roses, osage orange, and devil’s walking stick.
  • Bulbils are axillary buds that have become fleshy and rounded due to storage of food. They become detached from the plant, fall on ground and develop into a new plant.
  • Cladodes are green branches of limited growth (usually one internode long) which have taken up the functions of photosynthesis.

Regenerating hair follicle stem cells

Harvard University researchers have identified the biological mechanism of how chronic stress impairs hair follicle stem cells, confirming long-standing observations that stress might lead to hair loss.

In a mouse study published in the journal Nature, the researchers found that a major stress hormone causes hair follicle stem cells to stay in an extended resting phase, without regenerating the hair follicle and hair. The researchers identified the specific cell type and molecule responsible for relaying the stress signal to the stem cells, and showed that this pathway can be potentially targeted to restore hair growth.

"My lab is interested in understanding how stress affects stem cell biology and tissue biology, spurred in part by the fact that everyone has a story to share about what happens to their skin and hair when they are stressed. I realized that as a skin stem cell biologist, I could not provide a satisfying answer regarding if stress indeed has an impact -- and more importantly, if yes, what are the mechanisms," said Ya-Chieh Hsu, Ph.D., the Alvin and Esta Star Associate Professor of Stem Cell and Regenerative Biology at Harvard and senior author of the study. "The skin offers a tractable and accessible system to study this important problem in depth, and in this work, we found that stress does actually delay stem cell activation and fundamentally changes how frequently hair follicle stem cells regenerate tissues."

The hair follicle is one of the few mammalian tissues that can undergo rounds of regeneration throughout life, and has become a paradigm that informs much of our fundamental understanding of mammalian stem cell biology. The hair follicle naturally cycles between growth and rest, a process fueled by hair follicle stem cells. During the growth phase, hair follicle stem cells become activated to regenerate the hair follicle and hair, and hairs grow longer each day. During the resting phase, the stem cells are quiescent and hairs can shed more easily. Hair loss can occur if the hairs shed and the stem cells remain quiescent without regenerating new tissue.

The researchers studied a mouse model of chronic stress and found that hair follicle stem cells stayed in a resting phase for a very long time without regenerating tissues. A major stress hormone produced by the adrenal glands, corticosterone, was upregulated by chronic stress providing corticosterone to mice was able to reproduce the stress effect on the stem cells. The equivalent hormone in humans is cortisol, which is also upregulated under stress and is often referred to as the "stress hormone."

"This result suggests that elevated stress hormones indeed have a negative effect on hair follicle stem cells," Hsu said. "But the real surprise came when we took out the source of the stress hormones."

Under normal conditions, hair follicle regeneration slows over time -- the resting phase becomes longer as the animals age. But when the researchers removed the stress hormones, the stem cells' resting phase became extremely short and the mice constantly entered the growth phase to regenerate hair follicles throughout their life, even when they were old.

"So even the baseline level of stress hormone that's normally circulating in the body is an important regulator of the resting phase. Stress essentially just elevates this preexisting 'adrenal gland-hair follicle axis,' making it even more difficult for hair follicle stem cells to enter the growth phase to regenerate new hair follicles," Hsu said.

After establishing the link between the stress hormone and hair follicle stem cell activity, the researchers looked for the biological mechanism underlying the connection.

"We first asked whether the stress hormone was regulating the stem cells directly and checked by taking out the receptor for corticosterone, but this turned out to be wrong. Instead, we found that the stress hormone actually acts on a cluster of dermal cells underneath the hair follicle, known as the dermal papilla," said Sekyu Choi, Ph.D., the lead author of the study.

Dermal papilla is known to be critical for activating hair follicle stem cells, but none of the previously identified factors secreted from dermal papilla changed when stress hormone levels were altered. Rather, the stress hormone prevented dermal papilla cells from secreting Gas6, a molecule that the researchers showed can activate the hair follicle stem cells.

"Under both normal and stress conditions, adding Gas6 was sufficient to activate hair follicle stem cells that were in the resting phase and to promote hair growth," Choi said. "In the future, the Gas6 pathway could be exploited for its potential in activating stem cells to promote hair growth. It will also be very interesting to explore if other stress-related tissue changes are related to the stress hormone's impact on regulating Gas6."

These initial findings in mice need to be further studied before they can be safely applied to humans. Harvard's Office of Technology Development has protected the intellectual property relating to this work and is exploring opportunities for collaboration on its further development and eventual commercialization.

Last year, Hsu's group discovered how stress affects another type of stem cell located in the hair follicle, the melanocyte stem cells that regenerate hair pigment. The researchers found that stress activates the sympathetic nervous system and depletes melanocyte stem cells, leading to premature hair graying. Now with the new study, the two findings together demonstrate that although stress has detrimental impacts on both hair follicle stem cells and melanocyte stem cells, the mechanisms are different. Stress depletes melanocyte stem cells directly via nerve-derived signals, while stress prevents hair follicle stem cells from making new hairs indirectly via an adrenal-gland-derived stress hormone's impact on the niche. Because hair follicle stem cells are not depleted, it might be possible to reactivate stem cells under stress with mechanisms such as the Gas6 pathway.

Beyond the potential application of the Gas6 pathway in promoting hair growth, the study's results also have broader implications for stem cell biology.

"When looking for factors that control stem cell behaviors, normally we would look locally in the skin. While there are important local factors, our findings suggest that the major switch for hair follicle stem cell activity is actually far away in the adrenal gland and it works by changing the threshold required for stem cell activation," Hsu said. "You can have systemic control of stem cell behavior located in a different organ that plays a really important role, and we are learning more and more examples of these 'cross-organ interactions.' Tissue biology is interconnected with body physiology. We still have so much to learn in this area, but we are constantly reminded by our findings that in order to understand stem cells in the skin, we often need to think beyond the skin."


Modification of Stem

Since underground, they may seem like roots but they have characteristics of the stem, like the presence of nodes and internodes, scaly non-green leaves and buds. This modification serves two functions it acts as perennating structures by remaining leafless and dormant in winter but giving off aerial shoots under favourable conditions (next season) and Store food and become thick and fleshy.

Rhizome:

It is a prostrate, dorsoventrally thickened brownish stem, which grows horizontally under the surface of the soil. It shows distinct nodes and internodes. It shows distinct nodes and internodes. It bears scale leaves on nodes, It possesses terminal bud and axillary buds in the axil of each scale-leaf present at the node. Adventitious roots are present. e.g. Ginger (अद्रक), Turmeric (हलदी).

Stem Tuber:

Tubers are actually the swollen tips of ends of special underground branches swollen due to the storage of food as starch. The tubers show nodes and internodes. Nodes bear scale leaves with axillary buds, commonly called eyes. Under favourable conditions eyes sprout and produce aerial roots. Thus tubers help in vegetative propagation. They do not produce adventitious roots. e.g. Potato (आलू).

Bulb:

It is a condensed disc like an underground stem. The upper surface of the disc-like stem is conical and bears centrally placed apical bud and many concentrically arranged overlapping scale leaves. Scale leaves store food. When the scale leaves surround the apical bud in the form of concentric rings, it is called a tunicated bulb. The lower surface of the stem produces adventitious roots. e.g. onion (प्याज).

When the scale leaves partially surround the apical bud by overlapping each other, it is called a scaly bulb. The lower surface of the stem produces adventitious roots. e.g. garlic (लहसुन).

Corm:

It is condensed disc-like underground, fleshy, spherical stem with flattened base, It grows vertically, bears many scale leaves, distinct nodes and internodes, buds and adventitious roots. e.g. Saffron (केसर), yam (जिमीकंद), gladiolus.

Sub Aerial Modification Of Stem:

Stems are weak, therefore lie prostrate on the ground or may get partially buried in the upper layer of soil. The plants bearing such stems are called creepers. Their stems serve the function of vegetative propagation.

Runner:

The basal internodes of the bud elongate horizontally and trail along the soil carrying the bud to a distance from the mother plant where it gets fixed to the soil by means of adventitious roots and develops a new daughter plant. This branch carrying the bud is called a runner. It grows in all the direction and a single plant soon covers a large area by its pro­geny. e.g. Grass, Oxalis, Centella asiatica, strawberry, etc.

Stolon:

When a weak lateral branch which grows upwards then arches down to meet the soil, strike roots and produce daughter plants. The difference between the runner and stolon is, runner grows horizontally, while stolen grow obliquely upward and then arches to the ground. e.g. Mint (‘Pudina’), Jasmine.

Offset:

Like runner but thicker and shorter, grow for a short distance then produce cluster (rosette) of leaves above and adventitious roots below generally in aquatic plants. It is just like the runner, only it is shorter and thicker. e.g. water hyacinth (Eichhornia crassipes), pistia (Pistia stratiotes), water lettuce

Sucker:

An underground runner that grows horizontally for a distance under soil then emerges obliquely upwards, strikes roots and forms daughter plants. e.g. Chrysanthemum, Mentha arvensis, banana, pineapple etc.

Aerial Modification of Stem:

The whole stem or its part (axillary or terminal bud) gets modified to perform definite functions. It is a stem because they show characteristics like a) Arise in the axil of leaf b) Bear nodes and internodes c) may bear leaves, buds, flowers.

Stem Tendrils:

Stem or its branches get modified into green threadlike, spirally coiled leafless structures called tendrils which are meant for climbing. They twine around neighbouring objects and help weak plants to climb. These may be branched or unbranched. A scale leaf is always present at the point of branching of the tendril. e.g. Grapevine, cucumber, pumpkins, watermelon, etc.

Thorns:

These are straight, pointed, hard or woody structures sometimes they bear leaves, flowers or may be branched. Axillary buds of stems get modified into thorns (e.g. Citrus, Duranta, and Aegel). In Carrissa, terminal buds get modified into thorns. Thorns are used as organs of defence against grazing animals or climbing (e.g. Bougainvillea) and to check transpiration.

Phylloclade:

These are fleshy, green flattened or cylindrical branches of unlimited growth with nodes and internodes. The leaves are modified into spines or scales to check transpiration. This modification of stem is observed in plants growing in dry regions. The stem takes part in photosynthesis and stores water. e.g. Opuntia, Euphorbia, Casuarina, Cocoloba etc.

Cladode or Cladophylls:

It is a phylloclade with limited growth i.e. with only one or two internodes help in photosynthesis. These are a green cylindrical or flattened leaf-like branches. In Asparagus, the cladodes are one internode long and in Ruscus, the cladodes are two internodes long. They help in photosynthesis.

Bulbil:

These are modified vegetative or floral buds with stored food and meant for vegetative propagation. In Dioscorea, bulbils are condensed axillary buds while in Agava and lily the floral buds develop into bulbil. They detach to develop into a new plant.


Aerial Stem Modifications

The aerial stem modifications causes massive modification of the stem and they are difficult to be recognized as stems. For this reason they are called metamorphosed stems. In some plants the aerial stem is modified to perform a variety of special functions. The aerial stem modifications are as follows and usually play a definite function:

Thorn

Structural Characteristics:

The thorns is a hard, straight, and pointed structure.The axillary branches stop growing and get modified to straight, hard, pointed structures in such aerial stem modifications. They may bear small leaves flowers etc. and they have vascular traces.
Function :
They act as defensive organs and protect the plant from grazing animals.
Example: Carissa, Duranta, Citrus.

Phylloclade

Structural Characteristics:

A phylloclade is one of the aerial stem modifications that flattened stem of several internodes functioning as a leaf. Here the stem becomes flattened, fleshy, leaf-like structure. They can store water for future use. The nodes and internodes are not properly differentiated and the nodes are marked with the appearance of flowers and spines.
Function :
They help in the preparation of food by photosynthesis. They store food and water and the spines prevent loss of water by transpiration and also protect the plant from grazing animals.
<Example: Opuntia.

Cladode

Structural Characteristics:
A phylloclade of one or two internodes is called as a cladode. The cladode is a phylloclade with internodes. That is, the internodes in between two distinct nodes become leaf-like. It is photosynthetic, but not as fleshy as phylloclade and is devoid of spine.
Function :
The function of this type of aerial stem modifications is photosynthesis and storage of water.
Example: Asparagus (B. Satamuli).


30.2D: Stem Modifications - Biology

The stem is the main axis of the plant body arises from the plumule of an embryo. It bears branches, leaves, and flowers. The stem is characterized by its positively phototropic nature and the presence of nodes, internodes, and buds. On the basis of position, buds can be categorized into two types:

  • Apical or Terminal bud occurring at the apex or growing tip
  • Auxiliary or lateral bud occurring on the axil of leaf

Modifications of stem

Underground modifications of stem

In some herbaceous plants, the aerial part die off during unfavourable conditions and the reserve food materials are stored in their underground portion due to which they become thick and fleshy. These structures are although underground are considered as the modified stems because they have distinct nodes, internodes, scaly leaves and buds on them.

These underground modified stems can grow into the new plant with the coming of favourable conditions. Thus, they are also known as organs of perennation. Its types are discussed below:

Rhizome is underground, branched, dorsiventral horizontally growing modified stem having distinct nodes and internodes. Thin and membranous scaly leaves arise on the nodes. Apical as well as auxiliary bud arise on the rhizome which grows into new aerial shoots. Adventitious root arises from the lower surface of a rhizome. Eg Zingiber officinate (Ginger).

It is spherical, an underground structure having reduced conical stem known as disc in which the internodes are highly compressed and nodes closely arranged. Thick and fleshy scaly leaves arise on the nodes which store reserved food materials. Apical bud is enclosed by the scaly leaves and auxiliary buds are sometimes developed. Adventitious root arises from the lower surface of the stem. Eg Allium cepa(Onion), Allium sativum (Garlic).

It is underground swollen vertically growing modified stem having circular nodes and internodes. Thin membranous and brownish scaly leaves arise on the nodes. Single apical bud is prominent and auxiliary buds are sometimes developed. Adventitious roots arise normally all over the body of the corm. Eg Colocasia, Croccus sativus(Safron).

Tubers are swollen terminal portions of an underground stem which are covered with a thin corky sheath having lenticels(pores). Many depression known as eyes are present which represent nodes. Thin scaly leaves and buds arise on the eyes or nodes. Adventitious roots are absent. Eg Solanum tuberosum (Potato).

Sub-aerial modification of stem

The sub-aerial modifications are found in many herbaceous plants with a thin, delicate and weak stem. In such plants, a part of the stem lives underground whereas remaining part of the stem is aerial. These plants bear adventitious roots and aerial branches at nodes. Based on type of growth and part of plant that provides them, it is divided into four types:

The runner is a specialized weak stem that has long and thin internodes and the branches creep over the surface of the soil. The branches develop adventitious roots from the lower sides of each node. The aerial branches develop from the axil of the scale leaves at the nodes. The runner gives rise to new plants either from axillary or terminal buds. Eg Cynodon dactylon (Doobo), mint.

It is a thin layered branch which grows horizontally outward and bears nodes and internodes. It produces adventitious root at the point of contact with the soil. Leaves are distributed all over the stolon due to normal apical growth. Eg Strawberry

It is the lateral branch which develops from the axillary bud of an underground part of the stem. It grows obliquely and gives rise to leafy shoot. The adventitious root develops at the base of nodes of the underground part of a sucker. Eg Mentha arvensis (Mint), Chrysanthemum.

The offset is also known as condensed aquatic runners. It is weak, elongated, horizontal branches at one internode that arise in the axil of a leaf. An offset produces a tuft of leaves above and a cluster of the roots below. The offset may break off from the parent plant and acts as an independent plant. Eg Pistia(Water lettuce).

Aerial modifications of stem

Phylloclade:

It is the characteristic feature of some xerophytic plants. Phylloclade can be defined as green, swollen, cylindrical or flattened aerial modified stem of unlimited growth. It has many depressions known as Aereoles representing on the nodes on which reduced leaves or spines and buds are developed. Phylloclade store water in the form of mucilage and is covered by thick cuticle so that the plant can survive in a dry habitat. Eg Opuntia(Nagphani), Euphorbia royleana(Siundi)

Cladode is short, green aerial modified stem or branch of limited growth having single internode only. It can perform photosynthesis on itself because the leaves are modified into thin membranous scales. Eg Asparagus racemoscus (Kurilo).

Tendrils are thin, elongated, thread-like spirally coiled modified stem which may be branched or unbranched and provide support to the climbers. Tendrils arising from the axils of leaves are known as axillary tendrils. Eg: Passiflora (Passion flower). Tendrils arising on the internodes are known as extra-axillary tendrils. Eg Cucurbita (Gourd). Tendrils arising on the opposite of leaf are known as leaf-opposed tendrils. Eg Vitis vinifera(Grapevine).

Thorns are short pointed hard often straight aerial, modified stem arising from the axil of the leaf which may be branched or unbranched and may bear leaves or flowers. Eg Durant repens, Punica granatum(Pomegranate), Bougainvillea.

Bulbils are modified axillary buds which are meant for vegetative propagation of the plant. They develop from axillary buds, which become swollen and fleshy with food and drop off to form new plants. Eg Agave.

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Things to remember
  • The stem is the main axis of the plant body arises from the plumule of an embryo.
  • The stem is characterized by its positively phototropic nature and the presence of nodes, internodes, and buds.
  • Rhizome is underground, branched, dorsiventral horizontally growing modified stem having distinct nodes and internodes.
  • Phylloclade can be defined as green, swollen, cylindrical or flattened aerial modified stem of unlimited growth.
  • Cladode is short, green aerial modified stem or branch of limited growth having single internode only.
  • Phylloclade store water in the form of mucilage and is covered by thick cuticle so that the plant can survive in the dry habitat.
  • It includes every relationship which established among the people.
  • There can be more than one community in a society. Community smaller than society.
  • It is a network of social relationships which cannot see or touched.
  • common interests and common objectives are not necessary for society.

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Stem Modifications

To perform specialized functions the stem and the aerial branches get modified in different ways which is called as stem modifications. These modifications are of 3 kinds, viz., underground, sub-aerial and aerial. The sub-aerial modifications are also called metamorphosed stem because their structures are massively modified and sometimes cannot be predictable as stems.

Underground stem modifications

The underground stem modifications form are specialized stems, which are not positively phototropic, they belong to herbaceous plants, acquire various sizes and shapes and store food as carbohydrate in the form of starch. They keep the plant alive during unfavorable condition and with the initiation of favorable condition they produce the aerial shoot and spread by vegetative means. The general functions of underground stem modifications are:
[i] Storage of foodstuffs.
[ii] They act as per-enacting organs, surviving through unfavorable conditions.
[iii] They act as vegetative propagating organs.

The underground stem modifications are of the following types:

Stem-tuber

Characteristics of this type are as follows:

The tips of underground creeping branches become swollen and enlarged due to the accumulation of food materials. They are more or less round or oval in shape with compressed nodes and short internodes. The nodes are represented by eye-spots, which are depressions containing scale leaves and rudimentary buds remaining at the axils of scale-leaves and these eye-spots are spirally arranged on the tuber mostly remaining at the distal end. The apical bud is also present at the distal end of this form of stem modifications. The adventitious roots are absent on the ventral surface and it is colorless, and the outer epidermis dark brown to purple in color. Example: potato.

Characteristics of this type are as follows:
Stem Modifications of this type get the bulb-like structure with a central basal disc supporting the apical bud. The apical bud is protected by compressed layers of fleshy leaves and outer scaly leaves whereas the fleshy leaves store carbohydrate and may contain the axillary buds. The apical bud produces the flowering scape during the production of flowers. There is a crown of adventitious roots arising from the ventral surface of the disc and the fleshy leaves are arranged in closed concentric rings, proto-catechuic acid. Example: onion.
There are two types of bulbs, those are as follows:

Tunicate bulbs

It is the type of stem modifications where the scale leaves are with fleshy leaf bases, which are arranged in concentric manner surrounding the short, disc-like compressed stem. The outer leaves are dry, membranous and form a tunic-like structure. Example: tuberose, onion.

Scaly bulb (Naked bulb)

In this case the thick and fleshy leaf bases of scale leaves are loosely arranged and partly cover each other only at the margins and they are devoid of tunic. Example: garlic, lily.

Rhizome

Characteristics of this type of stem modifications are as follows:
These are the thick, fleshy, underground stem, which grows horizontally. The nodes and internodes are present, but the internodes are compressed and are covered by brown, papery, scaly leaves. The adventitious, axillary buds are present at the axils of scaly leaves, which account for uneven branching of the rhizome. The apices of the branches contain terminal buds, helping in apical growth and the terminal or axillary buds produce leafy shoots during the growing season which dry of later on, leaving behind a ring-like scar mark on the nodes. There -are slender, thread-like adventitious roots arising from the ventral surface of the rhizome' at the nodes. Example for horizontal rhizome: ginger, turmeric.
Sometime the rhizomes are short, stout, vertical and instead of growing horizontally, they grow vertically downwards. They are called root stocks. Example: arum, water-lily, banana.
Sometimes the horizontal rhizomes are thin and slender, with long internodes and are called sobole. Example of this stem modifications is the grass.

Characteristics of this type of stem modifications are as follows:
It is a stout, solid, fleshy, club-shaped underground modified stem and they grow vertically or peripherally in the superficial soil. The nodes and internodes are present, nodes are loosely placed and the internodes are reduced and fused to form a single structure. There is a large apical bud and many adventitious buds arising from the axils of scale leaves. The adventitious roots arise from the entire surface of the corm. The axillary buds produce new young corms, which bulges out from all over the corm and accounts for its irregular shape. Example: Amorphophallus.


Primary and Secondary Growth in Stems

Plants undergo primary growth to increase length and secondary growth to increase thickness.

Learning Objectives

Distinguish between primary and secondary growth in stems

Key Takeaways

Key Points

  • Indeterminate growth continues throughout a plant’s life, while determinate growth stops when a plant element (such as a leaf) reaches a particular size.
  • Primary growth of stems is a result of rapidly-dividing cells in the apical meristems at the shoot tips.
  • Apical dominance reduces the growth along the sides of branches and stems, giving the tree a conical shape.
  • The growth of the lateral meristems, which includes the vascular cambium and the cork cambium (in woody plants), increases the thickness of the stem during secondary growth.
  • Cork cells (bark) protect the plant against physical damage and water loss they contain a waxy substance known as suberin that prevents water from penetrating the tissue.
  • The secondary xylem develops dense wood during the fall and thin wood during the spring, which produces a characteristic ring for each year of growth.

Key Terms

  • lenticel: small, oval, rounded spots upon the stem or branch of a plant that allow the exchange of gases with the surrounding atmosphere
  • periderm: the outer layer of plant tissue the outer bark
  • suberin: a waxy material found in bark that can repel water

Growth in Stems

Growth in plants occurs as the stems and roots lengthen. Some plants, especially those that are woody, also increase in thickness during their life span. The increase in length of the shoot and the root is referred to as primary growth. It is the result of cell division in the shoot apical meristem. Secondary growth is characterized by an increase in thickness or girth of the plant. It is caused by cell division in the lateral meristem. Herbaceous plants mostly undergo primary growth, with little secondary growth or increase in thickness. Secondary growth, or “wood”, is noticeable in woody plants it occurs in some dicots, but occurs very rarely in monocots.

Primary and secondary growth: In woody plants, primary growth is followed by secondary growth, which allows the plant stem to increase in thickness or girth. Secondary vascular tissue is added as the plant grows, as well as a cork layer. The bark of a tree extends from the vascular cambium to the epidermis.

Some plant parts, such as stems and roots, continue to grow throughout a plant’s life: a phenomenon called indeterminate growth. Other plant parts, such as leaves and flowers, exhibit determinate growth, which ceases when a plant part reaches a particular size.

Primary Growth

Most primary growth occurs at the apices, or tips, of stems and roots. Primary growth is a result of rapidly-dividing cells in the apical meristems at the shoot tip and root tip. Subsequent cell elongation also contributes to primary growth. The growth of shoots and roots during primary growth enables plants to continuously seek water (roots) or sunlight (shoots).

The influence of the apical bud on overall plant growth is known as apical dominance, which diminishes the growth of axillary buds that form along the sides of branches and stems. Most coniferous trees exhibit strong apical dominance, thus producing the typical conical Christmas tree shape. If the apical bud is removed, then the axillary buds will start forming lateral branches. Gardeners make use of this fact when they prune plants by cutting off the tops of branches, thus encouraging the axillary buds to grow out, giving the plant a bushy shape.

Secondary Growth

The increase in stem thickness that results from secondary growth is due to the activity of the lateral meristems, which are lacking in herbaceous plants. Lateral meristems include the vascular cambium and, in woody plants, the cork cambium. The vascular cambium is located just outside the primary xylem and to the interior of the primary phloem. The cells of the vascular cambium divide and form secondary xylem ( tracheids and vessel elements) to the inside and secondary phloem (sieve elements and companion cells) to the outside. The thickening of the stem that occurs in secondary growth is due to the formation of secondary phloem and secondary xylem by the vascular cambium, plus the action of cork cambium, which forms the tough outermost layer of the stem. The cells of the secondary xylem contain lignin, which provides hardiness and strength.

In woody plants, cork cambium is the outermost lateral meristem. It produces cork cells (bark) containing a waxy substance known as suberin that can repel water. The bark protects the plant against physical damage and helps reduce water loss. The cork cambium also produces a layer of cells known as phelloderm, which grows inward from the cambium. The cork cambium, cork cells, and phelloderm are collectively termed the periderm. The periderm substitutes for the epidermis in mature plants. In some plants, the periderm has many openings, known as lenticels, which allow the interior cells to exchange gases with the outside atmosphere. This supplies oxygen to the living- and metabolically-active cells of the cortex, xylem, and phloem.

Example of lenticels: Lenticels on the bark of this cherry tree enable the woody stem to exchange gases with the surrounding atmosphere.

Annual Rings

The activity of the vascular cambium gives rise to annual growth rings. During the spring growing season, cells of the secondary xylem have a large internal diameter their primary cell walls are not extensively thickened. This is known as early wood, or spring wood. During the fall season, the secondary xylem develops thickened cell walls, forming late wood, or autumn wood, which is denser than early wood. This alternation of early and late wood is due largely to a seasonal decrease in the number of vessel elements and a seasonal increase in the number of tracheids. It results in the formation of an annual ring, which can be seen as a circular ring in the cross section of the stem. An examination of the number of annual rings and their nature (such as their size and cell wall thickness) can reveal the age of the tree and the prevailing climatic conditions during each season.

Annual growth rings: The rate of wood growth increases in summer and decreases in winter, producing a characteristic ring for each year of growth. Seasonal changes in weather patterns can also affect the growth rate. Note how the rings vary in thickness.


In some plants, aerial stems are converted to perform specific functions. These are the following types.

1. Stem Tendril

Spiral fibers are found in the place of axillary bud or apical bud which helps in climbing up. Example grapes (Grapevine, Vitis vinifera)

2. Stem thorn

This is the Modifications of the axillary bud. It is thick and spiny. Example Lime (Citrus) Bougainvillaea. Stem thorn work as protection from animals.

3. Phylloclade

Stems become fleshy or green and form a flat leaf-like structure. It does photosynthesis. Example Hawthorne (Opuntia). Interns and nodes are present on the stem.

4. Cladode

A phylloclade stem with single internode is called cladode. That is flattened, green and photosynthetic. Example Asparagus and Ruscus aculeatus.

5. Bulbil

The vegetative or flower buds collects food and gets swollen. These help in vegetative propagation. Examples include Agave and Yam.


Modification of Root, Stem and Leaf - Morphology of Plants, Class 11, Biology | EduRev Notes

MODIFICATION OF ROOT, STEM & LEAF

Radicle comes out/arise from the seed coat in the form of soft structure and move toward the soil. It develops and forms primary root.

TYPES OF ROOTS

 1 . Tap root : It develops from radicle which is made up of one main branch and other sub-branches. eg. : Dicots

 2 . Adventitious roots : In some plants after sometime the growth of tap root which arises from radicle stops and then roots develop from other part of plant which are branched or unbranched, fibrous or storage, are known as adventitious roots. eg. : Monocots

 

 

MODIFIED ROOTS

Tap and adventitious roots are modified in different forms to perform special functions are called as modified roots.

1 . Modified tap root for storage :

(i) Fusiform  oots /Spindle root - These root are thicker in the middle and tapering on both ends. In this type of roots root help in storage of food. Eg . :- Radish

(ii) Conical roots - These roots are thicker at their upper side and tapering at basal end eg. Carrot

(iii) Napiform - These roots become swollen and spherical at upper end and tapering like a thread at their lower end.  Eg. Turnip (Brassica rapa ), Sugarbeet (Beta vulgaris )

(iv) Tuberous root - Such roots do not have regular shape and get swollen & fleshy at any portion of roots. Eg. Mirabilis.

(v) Nodulated root - Nodules are formed on branches of roots by nitrogen fixing bacteria. (Rhizobium ).

Eg. Plants of leguminosae family (Papilionatae) - Pea

2 . Tap root modified for respiration

The plants which grow in marshy areas, scarcity of oxygen is found. The plants, which grow in this region some branches of tap root grow vertically upward and comes on surface of soil. These roots are called pneumatophores have minute pores called pneumathodes or lenticels by which air entered the plant and get oxygen for respiration. Eg. Rhizophora, Mangrove, Heritiera

Modification of adventitious roots :

(i) Tuberous adventitious root : When food is stored in these roots, they become swollen and form a bunch. Eg. Sweet potato (Ipomoea batatus)

(ii) Fasciculated roo ts - These are adventitious roots occuring in clusters and all of them are Swollen. (These roots have no definite shape ) Eg . Asparagus , Dahlia

(iii) Fibrous -  Roots are very thin and filamentous.  Eg. Grass, Wheat

(iv) Nodulose - In this type, tip s of roots swell up.  Eg. Melilotus , Curcuma amada

(v) B aded or moniliform - When root swell supl ike a bead at different places after a regular interval.  Eg. Vitis , Momord ica (Bitter gourd), Portulaca

(vi) Stilt roots or brace roots - When root arises from lower nodes and enter in side the soil and form a rope like structure, known as stilt roots   Eg. Maize , Sugarcane, Pandanus ( screw pine)

(vii) Prop root or pillar roots - When root arises from branches of plant and grows downward towards soil function as supporting stem for the plant. This type of roots are called prop root Eg. Banyan

(viii) Butress root - Such roots appear from the basal part of stem and spread in different directions in the soil. Eg. Terminalia

 (i x) Climbing roots - These roots arise from nodes and helps the plant in climbing Eg. Money plant (pothos ), Monstera, Betel (Piper betel), Black pepper

 (x) Respiratory root - When the quantity of oxygen is low in soil then some root comes out from the soil and helps in respiration.  Eg.Avicennia, Jussiaea

 (xi) Foliar root or Epiphyllous root : When roots arise from leaf are called as foliar roots Eg. Bryophyllum, Begonia

(xii) Sucking or haustorial roots or Parasitic roots : In parasitic plants, roots enter in the stem of host plant to absorbed nutrition from host. Eg . Dendrophthoe , Cuscuta, Viscum.

 (xiii) Annulated roots : If the swelling is in a series of ring on the roots. Eg. Ipecac Note : Some adventitious roots are also produced through branch cutting, when branches are put into the soil as in Rose, Sugarcane, Tapioca or when kept in water as in Coleus.

Stem is a part of plant which lies above from surface of soil i.e. it shows negative geotropic growth. It has nodes and internodes. Branches, leaf, flower bud and bracts are developed from nodes.

The main function of the stem is spreading out branches bearing leaves, flowers and fruits. It conducts water, minerals and photosynthates. Some stems perform the function of storage of food, support. protection and of vegetative propagation.

Forms of stem :

ف . Strong stem ( Erect stem) :

(a) Caudex : It is unbranched, erect, cylindrical stout stem and marked with scars of fallen leaves as in palms. Eg : Palm.

 (b) Culm : Stem is jointed with solid nodes  & hollow internodes. Eg : Bamboo (Gramineae).

2 . Weak stem : They are of three types :

 (i) Trailing - It is a weak stem that spreads over the suface of the ground without rooting at the nodes. (a) Prostrate or procum bent : If the stem trails on the ground and lie prostrate. Eg : Evolvulus and oxali s (Wood sorrel)

 (b) Decumbent : Stem trails for some distance and then tends to rise at its apex. E g : Tridax , Portulaca

 (c) Diffuse : When the branches of the stem are spread out in all directions on the ground.

Eg : Euphorbia, Boerhaavia

(ii) Climbers : Stem which attach the mselves to nearby object by means of some special devices like hooks, tendrils. Eg. Bougainvillea (by hook), wild pea (by tendrils)

 (iii) Creeping : The plant grows horizontally on the ground and gives off roots at each nodes.

Modification of stem : A - sub-aerial modification -These are creeping stem.

 (1) Runner -  When stem grows and spread on the surface of soil. Roots are developed at lower side and leaves from upper side from node Eg. Cynodon dactylon (Doob grass), Oxalis .

(2) Stolon - In it branches develop from the lower part of the main stem growing some distance like arch and finally touching ground to give rise to new shoot. Eg . Fragaria (Wild strawberry) , Jasmine, Peppermint

 (3) Sucker - In it the main stem grow in the soil but branches develop from nodes and comesout from the soil.

Eg. Mint , Pineapple, Chrysanthemum.

 (4) Offset - Generall y thes e occur in aquatic plants which have fragile stem. Internodes o f offset are sm all & thicker. It is also known as aquatic runner. Eg. Pistia, Eichhornia

B Underground modificationThis type of modification occurs generally for food storage and vegetative propagation.

 (1) Tuber - The tips of branches become swollen in the soil. Eyes are found on them which are axillary buds and covered with scaly leaves. Eg. Potato

 (2) Rhizome - It is fleshy and horizontally found below in soil small nodes and internodes are found which are covered by scaly leaves.  Eg. Ginger, Turmeric, Canna, Waterlily

 (3) Corm - It is condensed structure which grow vertically under the soil surface. Eg. Colocasi a, Alocasia , Zaminkand, Saffron.

 (4) Bulb - This stem has disc like structure and surrounds with numerous fleshy scaly leaves. Many roots arise from its base.  Eg. Onion, Garlic.

C - Aerial modification

 (1) Stem tendril - In this type axillary bud forms tendril in place of branches and helps in climbing of those plants whi ch have weak stem. Eg.Grapes, Passiflora, Cucumber , Pumpkins, Watermelon

 (2) Phylloclade - Stem is modified into a flat (Opuntia), fleshy Cylindrical (Euphorbia)and green leaf like structure and carries out photosynthesis like leaf.  The leaves are modified into spines Eg. Opuntia , Euphorbi a, Casuarina.

 (3) Prickle and hooks - These developed only from cortex and epidermis and found at nodes or internodes. It helps in climbing. It is exogenous in origin. E g. Rose, Smilax

 (4) Stem thorn/ Thorn - It is developed from axillary bud or terminal bud of the stem. It may bear leaves, flowers. It is endogenous in origin. Eg. Carissa(Karonda), Bougaivillea, Pomeg ranate, Citrus

The leaves develop from the nodes. Their main function is photosynthesis and food making axillary buds are found in its axil. Leaves originate from shoot apical meristems and are arranged in an acropetal order.

Leaf is divided into 3 main parts :

(1) Leaf base - The part of leaf attached to stem

 (2) Petiole - The part of leaf connecting the lamina with the branch or stem, petiolated or stalked leaves are known as petiolate and when petiole or stalk is absent then leaves are called sessile. In Eichhornia petiole swells and in citrus it is winged.

(3) Lamina (Leaf blade ) - It is a b roa d an d f lat tene d p art of lea f.  Its main functions are photosynthesis and transpiration.

Stipules : Leaves of some plants have lateral appendages on either side of leaf base, known as stipules.

If stipules are present in leaf it is called stipulated leaf,  if it is absent then leaf is called ex-stipulated.

Stipules are of various types

1 . Free lateral - The y are in dependen tly pres ent on b oth side s of lea f base. Eg. Hibiscus rosasinensis (China rose).

2 . Interpetioler - When two leaves are meet oppositely at the node then two nearest stipules of adjacent leaf joint with each other.  In this way only two stipules of two leaves are found in place of four  Eg. Ixora.

3 . Intrapetioler - In this type both stipules of a single leaf join with eac h other to form a single stipule.Eg : Gardenia

4 . Foliaceous - These type of stipules form a leaf like structure. Eg . :  Pea

5 . Scaly - Stipules are dry , small and paper like. E g : Desmodium

6 . Spiny - Stipules modified into spines. Eg. Zizyphus(Ber)

7 . Ochrea te - When both stipules of leaf combine together and form a tube like structure, it is called ochreate. Eg. polygonum

8 . Adnate - Both stipules are joint with petiole. Eg. Rose.

ى. Tendrillar -  Stipules are modified into tendrils like structure.  Eg. Smilax

Types of Leaves

1. Foliage leaf - They are usually green coloured and their main function is photosynthesis.

ق. Cotyledonary leaf - This leaf comes out during germination and helps in nutrition untill the first leaf is not formed.

3. Scale leaf - Such leaves are usually dry membrane like and they cannot perform photosynthesis.

4. Bract - Bract are the leaves which contain flower in their axil.

5. Bracteole - These are leaf like structure found on pedicel.

6. Floral leaf - Sepals, petals , stamen and carpel are found in flower which are included in this type ofleaf.

Note : Perianth : In some flowers, Calyx and Corolla are not distinct and are termed as Perianth, and unit of perianth is called tepal. Eg. Lily

Duration of Leaf :

ف. Persistent/ Evergreen - Leaves of such plants are found in all season and do not (fall) shed combindly. Eg. Pine, Saracaindica, Datepalm.

ق. Deciduous - All leaves of such plants shed a t the same tim e Eg. Azadirachta, Fi cus

3. Caducous - Leaves shed as the bud formation takes place. Eg : Rose

1. Cauline leaves - When the leaves are found on node of stem, then they are called cauline leaves.

2. Ramal leaves - When leaves are found on branches, then they are called ramal leaves.

3. Radical leaves - During favourable season ,leaves develop from the nodes of under ground stem and seem that they are developing from roots. This type of leaves are known as radical leaves.

VENATION OF LAMINA

The arrangement of veins and veinlets in leaves (Lamina) is known as venation. It is of 2 - types

(1) Reticulate. It is found in dicots. Exception – Calophyllum(It has parallel venation)

(2) Parallel. It is found in monocots. Exception – Smilax (It has reticulate venation)

ف. Reticulate venation - In this type of venation many veins are divided into various branches (veinlets) and form a net like structure.

Reticulate venation is o f 2- typ es

(a) Unicostate or pinnate - This type of venation is having only one principal vein or midrib that give off many lateral veins which proceed toward margin and apex of lamina of the leaf and form a network. Eg. : Mango, Guava, Peepal.

(b) Multicostate or palmate - In this type of venation many principal veins arising from the tip of petiole and proceed upward, this is again of two types –

(i) Multicostate divergent - Many principal veins arising from the tip of petiole diverge from one another towards the margin of leaf blade eg. Cotton, Castor, Cucurbita.

(ii) Multicostate convergent - Many principal veins arising from the tip of petiole. At the base of leaf they are closely arranged but diverge from one another in middle part and converge towards the apex of leaf. Eg. : Camphor, Zizyphus, Tejpat, China rose , Plum.

2 . Parallel venation - In this type of venation, all veins run parallel to each other and they donot form network.

They are of 2 types

( 1 ) Unicostate or pinnate : This type of pattern having only one principal vein, that gives off many lateral veins, which proceed toward the margin of leaf blade in a parallel manner but they donot have veinlets. Eg. Banana, Ginger, Canna

( 2 ) Multicostate or palmate : Having many principal veins arising from the tip of the petiole and proceeding upwards.

(a) Multicostate divergent : Many p rincipal vei ns arising from the tip of petiole and diverge tow ard the margin of leaf. They donot divide into veinlets and do not form network. Eg. : Coconut, Fan palm.

(b) Multicostate convergent : Many principal veins arising from the tip of petiole run in a curved manner in lamina and converge towards the apex of leaf blades. Eg. : Wheat, grass, sugar–cane, Bamboo

Modification of leaves - Whe n leaf is modified in different structure. It is called m odification of leaves.

(1) Leaf tendril - In this, whole leaf is modified into a wire like structure which is called leaf tendril Eg. Lathyrus aphaca (wild pea)

(2) Leaf spine - Leaves or any part of leaflet are modified into pointed spine.  Eg. Opuntia, Aloe,Argemone.

(3) Leaf scale - In this leaves bec ome thin, dry and form a membrane or paper like structure and serve to protect axillary buds as in Ficusand Tamarix, Ruscus or store food and water as in onion.

(4) Leaf pitcher - Leaves o f some plants are modified to pitcher shape . Eg . Nepen thes,Dishidia

(5) Leaf bladder -  In some plant, leaves are modified into bladder li ke structure eg. Utricularia

(6) Leaf Hooks - In some plants terminal leaf lets are modified into curved hooks for helping the plant in climbing.Eg. Ca t' s nail (Bignonia unguis - cati ).

(7) Phyllode - In it petiole becomes flat structure and function as normal leaf. Eg. Australian acacia

(8) Leaflet tendril - Whe n leaflea t is modif ied into tendril like structure than it is called leaflet tendril. Eg . : Pisumsativum(Garden Pea), Lathyrus odoratus (sweet pea).

Note : Dionaea (Venus flytrap) is insectivorous plant and they also have modified leaves.

Simple and Compound Leaf

( 1 ) Simple Leaf : A leaf which may be incised to any depth, but not up to the midrib or petiole then this type of leaf called simple leaf.

( 2 ) Compound leaf : A leaf in which the leaf blade is incised up to the midrib or petiole, thus dividing it into several small parts, known as leaf lets.

It has two types :

 (A) Pinnately compound leaf : In this type of leaf mid rib is known as rachis. Leaf lets are arranged on both sides of rachis. E g. Neem It is of four types

(i) Unipinnate : In this type of leaf divison occurs only once and leaf lets are directly attached on both sides of rach is. If the number of leaflet is even , then leaf is known as pa ri pin nat e. E g . : Cassia fistula, SesbaniaIf the number of leaflet is odd, it is known as imparipinnate. Eg. : Rose, Neem

(ii) Bipinna te : A twice pinnate compound leaf. Eg . : Acacia, Gulmohar, Mimosa

(iii) Tripinnate : A thrice pinnate compound leaf. Eg . Moringa

(iv) Decompound : A compound leaf, which is more than thrice pinnate. Eg : Carrot, Coriander

(B) Palmate compound : In this type incision of leaf are directed from leaf margin to apex of petiole and all leaflets are attached on the upper end of petiole. Eg. Silkcotton.

It is of following types :

 (i) Unifoliate : When single leaf let is found. Eg . : Lemon

 (ii) Bifoliate : W hen two leaf lets are present. Eg . : Bauhinia , Regnelidium

(iii) Trifoliate : W hen three leaf lets are attached . Eg. : Oxalis, Aegle.

(iv) Tetrafoliate : When four leaf lets are attached to the petiole. Eg . : Marsilea

(v) Multifoliate : When more than four leaflet are found, then leaf is called multifoliate  palmate compound leaf. Eg. Silkcotton.

Shape of lamina :

1. Acicular : Lamina is long and pointed, like a needle. Eg. Pine .

ق. Lanceolate : In this type lamina is pointed or narrower at the ends while broader in the middle. Eg. Bamboo, Nerium

3. Linear : The lamina is long and narrow having parallel margins. Eg. Grass

4. Ovate : In this type lamina is egg–shaped having broad base with slight narrow top. Eg. Ocimum , Banyan , China rose

5. Cordate : Its shape is like a heart. Eg. Betel.

6. Oblong : Long and broad lamina. Eg. Banana.

7. Sagittate : The lamina is triangular in shape. Eg. Sagittaria .

8. Spathulate : The lamina is broad spoon shaped. Eg. Calendula .

9. Orbicular or Rotund : In this type the lamina is spherical. Eg. Lotus.

10. Elliptic al or Oval : In this type the middle part of lamina is broad while the ends are narrow and oval. E g. Guava, Jamun

11. Oblique: In this type midrib divides, lamina into two unequal halves. Eg. Begonia , Neem.


Explain Modifications of Stem

The stem may not always be typically like what they are expected to be. They are modified to perform different functions (Figure). Underground stems of potato, ginger, turmeric, zaminkand, Colocasia modified to store food in them. They also act as organs of perenation to tide over conditions unfavorable for growth. Stem tendrils which develop from axillary buds, are slender and spirally coiled and help plants to climb such as in gourds (cucumber, pumpkins, watermelon) and grapevines.

Axillary buds of stems may also get modified into woody, straight and pointed thorns. Thorns are found in many plants such as Citrus, Bougainvillea They protect plants from browsing animals. Some plants of arid regions modify their stems into flattened (Opuntia), or fleshy cylindrical (Euphorbia) structures.

Figure: Modifications of stem for: (a) storage (b) support (c) protection (d) spread and vegetative propagation

They contain chlorophyll and carry out photosynthesis. Underground stems of some plants such as grass and strawberry, etc., spread to new niches and when older parts die new plants are formed. In plants like mint and Jasmine, a slender lateral branch arises from the base of the main axis and after growing aerially for some time arch downwards to touch the ground.

A lateral branch with short internodes and each node bearing a rosette of leaves and a tuft of roots are found in aquatic plants like Pistia and Eichhomia. In banana, pineapple and Chrysanthemum, the lateral branches originate from the basal and underground portion of the main stem, grow horizontally beneath the soil and then come out obliquely upward giving rise to leafy shoots.



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