Gymnosperms have their history since the time of the Paleophyte. It is for the ancient history of plants that a wide distribution of spore is characteristic. They were the main representatives of the flora. The Late Permian period brings serious changes to the plant world: the era of coal marshes with their flora is being replaced by a time of completely different plants. Gymnosperms develop especially rapidly and occupy leading positions during the Mesozoic.
The emergence of this group of plants was a progressive step in the evolutionary process. The pre-existing fern in the process of reproduction required an aquatic environment. The spore, getting from sporangia into moist soil, sprouted a seedling with genital organs. If a plant forms different spores during development, then two growths develop from them: one with female and one with male organs. Further, in order for the fertilization process to take place, archegonia - the female reproductive cell - must connect to the male reproductive cell, and the presence of water is a prerequisite. It is this argument that indicates that preexisting ferns and psilophytes descended from aquatic plants. The same feature significantly limited the distribution of plants - growth could occur only in places with high humidity.
The new group has lost its dependence on the presence or absence of water. And the reproduction of gymnosperms occurs as follows: the wind carries pollen, and the development of the seed occurs directly on the maternal specimen. But this is not the only advantage of this group. If the spore is only 1 cell, the supply of nutrients is limited, then the seed, as a multicellular formation, is provided with a supply of substances vital for the development of the embryo. In addition, the presence of seed flakes for the embryo is a natural reliable protection against adverse environmental influences. These new structural features were important for the evolution of the flora; gymnosperms received improved conditions for reproduction and conservation.
Today, these plants are distinguished by scientists in a separate class. And the whole history of their development proves that angiosperms evolved precisely from gymnosperms. All representatives of this group (gymnosperms) are divided into four departments: ginkgoid, opiate, cypress, coniferous.
What unites this group? What are the symptoms of gymnosperms? All of them are shrubs or trees, often very large. Some, highly branched, are distinguished by a large number of small leaves (can be scaly). Others, on the contrary, are lightly branched, but with huge cirrus leaves. A xylem without vessels, and a phloem without a companion cell is another feature characteristic of most gymnosperms. They are heterogeneous. The shape, size and structure of micro and macro sporophylls vary greatly. Free growth on an ordinary shoot - in a primitive seed fern, on strobils (shortened shoots) - in any other gymnosperm plant.
Pollen pores or dust particles are microspores, the place of development of which is the pollen bag. It is them that the wind carries to the female gametophyte. In the ovule, the megaspore is already developing. When fertilized with a male gamete, the ovum becomes a seed. A distinctive feature of female gametophyte is resistance to dehydration. The seed is surrounded by a nutrient reserve, which the zygote uses in the process of germination, and until the onset of favorable conditions, the seed remains in a calm state. The fruit does not form, but the seed is capable of developing various adaptations.