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Gymnosperms


The gymnosperms (from the Greek Gymnos: 'nu'; and sperma: 'seed') are terrestrial plants that live preferably in cold or temperate climate environments. This group includes plants such as Pine trees, at redwoods and the cypress trees.

Gymnosperms have roots, stem and leaves. They also have reproductive branches with modified leaves called strobiles. In many gymnosperms, such as pines and sequoias, strobiles are well developed and known as cones - which gives them the classification in the group of conifers.


Cones or strobiles

Coniferous forests of temperate regions are rich in gymnosperm trees. In Brazil, we highlight the Araucarias Forest of the South of the country.

There is seed production: they originate in female strobiles. However, gymnosperms do not produce fruit. Your seeds are "bare", meaning they are not enclosed in fruits.


Araucarias


Cyca

Gymnosperms are also the Cycas, popularly known as palm or sago palm, common in some places in Brazil. The trunk is also usually thick, the leaf is similar to that of palm trees, but is much more rigid.

Gymnosperm Reproduction - Haplodiplobion Cycle in Conifers

Let's use the parana pine tree (Araucaria angustifolia) as a model for explaining gymnosperm reproduction. In this plant the sexes are separated: the one with male strobiles does not have female strobiles and vice versa. In other gymnosperms, both types of strobilus may occur in the same plant.

There are two types of strobiles, one large and one small, and as a consequence there are two types of sporangia and spore. In larger female strobiles, each sporangium, called an egg, produces by meiosis a megaspore (or macrospore). The megaspore is trapped in the sporangium and is not released, as is the case with pteridophyte spores. Developing inside the egg the megaspore gives rise to a female gametophyte. In this gametophyte arises archegons and within each one of them one the sphere (which is the female gamete).

In smaller male strobiles, each sporangium - also called pollen bag - produces by meiosis, numerous microspores. Developing within the pollen sac, each microspore originates a male gametophyte, also called pollen grain (or young male gametophyte). The rupture of pollen sacs releases numerous light pollen grains with two winged lateral expansions. Carried by the wind, they can reach the eggs found in female strobiles. The process of transporting pollen grain (do not forget that they represent male gametophytes) constitutes pollination, which in this case occurs by wind.

Each pollen grain, attached to an opening in the egg, begins a growth process that culminates in the formation of a pollen tube, corresponding to one adult pollen grain (adult male gametophyte). Inside the pollen tube there are two haploid gametic nuclei, corresponding to the pteridophyte anterozoids. Only one of the gametic nuclei fertilizes the oosphere, generating the zygote (the other gametic nucleus degenerates). Repeatedly dividing by mitosis, the zygote eventually gives rise to an embryo, which plunges into the maternal tissue corresponding to the female gametophyte.

After fertilization and formation of the embryo, the egg becomes a seed, which is a structure with three components: a house (also called an integument), an embryo and a haploid maternal tissue, which is renamed. endosperm (or primary endosperm) by accumulating reserve substances that will be used by the embryo during germination. Seed dispersal, under natural conditions, may occur by wind, in the case of the common pine, or with the help of animals (blue jackdaws or squirrels) as with the pine nuts of the parana pine.

Therefore, when comparing coniferous gymnosperms with pteridophytes, the following novelties can be cited: egg-producing strobiles (which will then be converted into seeds), pollen grain producing strobiles, pollination, pollen grain differentiation in pollen tube and finally the independent fertilization of environmental water (This type of fertilization is known as sifogamy). Note that coniferous trees represent the enduring generation, the sporophyte, and the gametophytes are small and short-lived.