Life history of gymnosperms
Now take pine plants as an example to introduce the life history of gymnosperms.
1. Life history of sporophytes The sporophytes of pine plants of gymnosperms are perennial woody plant bodies, uniaxial and branched, its main trunk is upright and upward, the wheel-like order of branches grows on the trunk, and the branches grow into short branches; There are true middle columns, and more than 90% of the middle columns are composed of tubular cells, with scaly leaves on long branches and short branches in the axilles.
The spore leaf bulbs of the genus Pinus are monosexual. There are round sporangia on the back of the small spore leaves, and the sporangia are spore mother cells, which form four spores after two consecutive divisions, the spores have an outer wall, and the outer wall bulges inside and outside to form air sacs, with the help of this structure the seeds can be spread out by wind.
Large spore leaf bulbs are born on the top branches, composed of bracts and bead scales, generally these two parts in the pine family are completely separated, the bracts have 2 ovules at the base, the ovules are composed of beads surrounded by the heart of the bead, the heart of the bead is the large sporangia, inside is the female gamete mother cell, after two consecutive divisions to form 4 female gametes, that is, eggs, only the cell on the junction can combine with the male gametes for fertilization, the other three will degenerate.
2. Male gametophytes The male gametophytes of the genus Pinus are severely degenerated and consist of only a few cells. The process is like this, first small spores, that is, pollen grains in the monocytes, which are the first cells of male gametes; This small spores germinate in the sporangia and undergo a division into two cells, one protoleaf cell and one embryonic cell, the protoleaf cell only plays a trophic function, the embryonic cell divides again, producing a second protolophyte cell nucleus spermatozoa protocell, spermatozoa protocells divide again to form a tube cell and a germ cell, can a mature male gametophyte actually contain two protolophyte cells, a tube cell and a germ cell These four cells.
3. Female gametophytes The female gametophytes of the pine genus are developed from large spores, which are the first cells of female gametophytes, this cell performs free nuclear division, forming a large number of nuclei distributed in the cytoplasm, but does not produce cell walls, until a certain stage the nucleus is large enough to form a cell wall, while the bead pore end cells expand into cervical oviposer primitive cells, this cell continues to divide to form cervical oocytes, a mature female gamete has 2-7 cervical ovigates and a large number of endosperm.
4. Pollination and fertilization Pollination is generally carried out in late spring, but fertilization is completed 13 months after pollination is completed, which is the cause of delayed gamete development in females. First of all, the central axis of the large spore leaf is elongated, the bract scales and bead scales are opened, the back of the small sporangia is opened, the pollen grains in the male gamete stage are spread out by wind, and the mucus secreted by the bead hole of the female gamete sucks up the pollen grains, and after drying up, the pollen grains are sucked into the ovule, and the bead scales are reclosed. After the pollen grain enters the ovule, the cell divides into tubular cells and somatic cells, the tubular cells develop into pollen tubes, somatic cells continue to divide into two sperm, 13 months later, the pollen tube extends into the female gametocyst is the cervical oocyst, until it reaches the egg cell, and 2 sperm and other cells are released, only the functional sperm and egg combine to form a fertilized egg, which is fertilization.
5. Embryonic development and seed formation
The embryonic development of the genus Pine is very complex, including four stages: the embryo stage, the embryo selection stage, the histological stage of the embryo, and the maturation stage of the embryo. The protoembryonic stage begins with the division of the fertilized egg to the formation of the protoembryo, undergoing free nucleus classification, cell wall production, and protoembryo formation; The main characteristics of the embryonic selection stage are embryonic peduncle phylogeny and multiple fetal phenomena; In the tissue differentiation stage of the embryo, the embryo further develops into a cylinder, the proximal axis of the cylinder is differentiated into the root end and root crown, and the distal axis is differentiated and developed into the cotyl, germ and cotyledon; The maturation embryonic stage is the result of continued differentiation and multi-layering of the parts of the embryo produced after the selection stage.
The seed is composed of the embryo, endosperm and seed coat. These three parts of the seeds of gymnosperms are the products of three different generations, of which the embryo is the product of the new generation; The endosperm is the old female gametophyte generation; The seed coat is an old sporophyte.
After fertilization, the large spore leaf bulb continues to develop, and the bead scales are lignified into seed scales, and the seed scales develop the scale shield, the scale navel and the wings of the seed, and the wings help the seeds to spread by wind; After the seeds begin to germinate, first the radicle sticks out of the bead hole to develop into roots and lateral roots, the cotyl and cotyledons absorb the nutrients of the endosperm and continue to develop, to a certain stage, the cotyledons and cotyledons extend out of the ground, the cotyledons develop into stems, cotyledons develop into the first leaves, and then the stem branches and grows leaves.
In fact, the life history of pine plants is very long, from spring flowering to the next autumn seed maturity has 18 months, and even if you count from the autumn flower primordium of the year before flowering, it takes three years, 26 months.
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