Gymnosperms

Gymnosperms have their history from the time of the paleophyte. It is for the ancient history of plants characterized by widespread spore. They were the main representatives of the flora. The period of late Permian brings to the plant world a major change: the era of Carboniferous swamps with their flora is replaced by the time of completely different plants. Gymnosperms especially develop rapidly and occupy leading positions in the Mesozoic period.

The emergence of this group of plants has become a progressive step in the evolutionary process. The fern existing in the process of reproduction required an aqueous medium. Spore, getting from sporangia to moistened soil, sprouted sprout with genitals. If the plant develops different spores during development, then two sprouts develop: one with the female, and the other with the male organs. Further, in order for the process of fertilization to take place, archegonia - the female germ cell - must be connected to the male germ cell, while the presence of water is a prerequisite. It is this argument that indicates that the ferns and psilophytes that existed earlier originated from aquatic plants. This same feature significantly limited the spread of plants - the growth could occur only in places with high humidity.

The new group lost its dependence on the presence or absence of water. A reproduction of gymnosperms occurs as follows: the wind carries pollen, and the development of the seed occurs directly on the parent specimen. But this is not the only advantage of this group. If the dispute represents only one cell, the supply of nutrients is limited, then the seed, as education is already multicellular, is provided with a supply of substances vital for the development of the embryo. In addition, the presence of seed scales for the embryo is a natural reliable protection against the adverse effects of the environment. These new structural features were important for the evolution of the flora; Gymnosperms have improved conditions for reproduction and preservation.

Today, these plants stand out as scientists in a separate class. And the whole history of their development proves that angiosperms originated precisely from gymnosperms. All representatives of this group (gymnospermous plants) are divided into four divisions: ginkgoed, yew-like, sagovnikovidnye, coniferous.

What unites this group? What are the signs of gymnosperms? All of them are bushes or trees, very often of enormous size. Some, strongly branched, are distinguished by a large number of small leaves (may be scaly). Others, on the contrary, are weakly ligated, but with huge pinnate leaves. Xylem without vessels, and phloem without a companion cell is another characteristic characteristic of most gymnosperms. They are multisporous. The shape, size and structure of micro- and macrosporophylls vary greatly. Free growth on the usual shoot - in a primitive seed fern, on strobila (short shoots) - in any other gymnosperm plant.

Pollen pores or motes are microspores, the development of which is a pollen bag. It is they who are borne by the wind to the female gametophyte. The megasore is already developing in the ovule. When fertilizing a male gamete, the ovule becomes a seed. A distinctive feature of the female gametophyte is its resistance to dehydration. The seed is surrounded by a nutritional reserve, which the zygote uses in the process of germination, and until the favorable conditions the seed remains in a calm state. The fetus is not formed, but the seed is capable of developing various adaptations.