gymnosperms trace their history from the time of paleofita.It was for the ancient history of the plant is characterized by widespread spore.They were the main representatives of the flora.Period of Late Permian flora brings major changes: replacing coal era swamps with their flora comes time completely different plants.Gymnosperms especially booming and occupy leading positions in the period of the Mesozoic.
appearance of this group of plants was a progressive step in the evolutionary process.Preexisting fern in the process of reproduction required aqueous medium.Spore, falling from the sporangium in moist soil, sprouting prothallia with the sexual organs.If the plant is in the development of different forms spores that are developing two prothallia one with women and one with men's bodies.Further, the process of fertilization to occur, archegonium - female germ cells - must connect to the male germ cells, and the prerequisite is the existence of water.It is this argument indicates that preexisting ferns and psilophytes evolved from aquatic plants.This is particularly important to limit the spread of the plant - vegetation could only occur in places with high humidity.
new group lost the dependence on the presence or absence of water.A reproduction of gymnosperms is as follows: the wind carries the pollen and seed development going on right on the maternal copy.But this is not the only advantage of the group.If the dispute is only one cell, the supply of nutrients is limited, then the seed of education has multicellular, ensured supply of substances that are vital in the development of the embryo.In addition, the availability of seed scales to the fetus is a natural reliable protection against adverse environmental impact.These new structural features were important to the evolution of the flora;gymnosperms obtain improved conditions for breeding and conservation.
Today these plants are distinguished scientists in a separate class.And the whole history of their development proves that it came from the angiosperms gymnosperms.All members of this group (gymnosperms) are divided into four sections: ginkgovidnye, gnetophyta, sagovnikovidnye, conifers.
What unites this group?What are the signs of gymnosperms?All of them are shrubs or trees, often of great size.Some, silnovetvyaschiesya, different lots of small leaves (can be scaly).Others, however, slightly branched, but with huge feathery leaves.Xylem without vessels, and phloem cells without companion - another characteristic of the majority of gymnosperms.They are heterosporous.The shape, size and structure of micro and makrosporofill very different.Free escape normal growth - a primitive fern seed on strobilae (short shoots) - any other gymnosperms.
Pollen pores or dust particles - is microspores, the place of which - the pollen sack.That wind carries them to the female gametophyte.The ovule is being developed already megaspores.When the male gamete fertilization ovule becomes the seed.A distinctive feature of the female gametophyte - resistance to dehydration.The seed is surrounded by a nutrient reserve that uses the zygote during germination and favorable conditions before the seed is in a calm state.The fruit is not formed, but the seed is able to develop various devices.
