Spermatophytes

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A spermatophyte (lit. 'seed-bearing plants'; from Ancient Greek σπέρματος (spérmatos) 'seed', and φυτόν (phytón) 'plant'), also known as phanerogam (taxon Phanerogamae) or phaenogam (taxon Phaenogamae), is any plant that produces seeds, hence the alternative name seed plant. Seed plants are distributed all over the world and are the highest group in the plant kingdom. All seed plants have two basic characteristics, that is, there are vascular tissues in the body—phloem and xylem, which can produce seeds and reproduce with seeds.

Seed plants can be divided into gymnosperms and angiosperms. The seeds of gymnosperms are naked, and the outer layer is not covered by the pericarp. Seeds of angiosperms are covered with pericarp.

Closely related to the emergence of seeds is the production of pollen tubes, which send sperm to the egg, so that the very important link of fertilization is no longer limited by the environment - water. Their sporophytes are well-developed, highly differentiated, and absolutely dominant; on the contrary, the gametophytes are extremely simplified and cannot live independently without the sporophytes. Seeds were first produced in gymnosperms of the order Pteridophyta, among which the most primitive fossil seed ferns were found in Upper Devonian strata. Both seed plants and ferns have alternation of generations.

Seed formation: The structure of the seed includes three parts: embryo, endosperm and seed coat, which are developed from fertilized egg (zygote), fertilized polar nucleus and integument respectively. The nucellus part of most plants is absorbed and utilized during the seed formation process and disappears. There are also a few species of nucellus that continue to develop until the seed matures and become the outer endosperm of the seed. Although the size, shape, and internal structure of different plant seeds are quite different, their development process is similar.

Embryo development

The embryo in the seed develops from the zygote, which is the first cell of the embryo, after the egg has been fertilized. After the egg cell is fertilized, it produces a cell wall of cellulose and enters a dormant state.

A zygote is a highly polarized cell whose first division, usually lateral (with rare exceptions), becomes two cells, one near the micropylar end, called the basal cell, and the other distal, called apical cells. The apical cell will become the precursor of the embryo, while the basal cell is only vegetative, not embryogenic, and will become the embryonic stalk later. There is a plasmodesmata communication between the two cells. This cell heterogeneity is determined by the physiological polarity of the zygote. Embryos that do not appear in the pre-differentiation stage are called proembryos. The process of developing from proembryo to embryo differs between dicots and monocots.

Development of the Dicotyledonous Embryo

The development of dicot embryo can be illustrated by shepherd's purse as an example. After a short period of dormancy, the zygote splits horizontally into basal cells and apical cells unevenly. The basal cells are slightly larger and undergo continuous transverse divisions to form a row of stalks consisting of 6-10 cells. The top cells first undergo a second longitudinal division (the second division plane is perpendicular to the first one) to become 4 cells, that is, the tetrad stage; then each cell divides horizontally once again to become a spheroid of 8 cells , that is, the octant period. Each cell of the octad first undergoes a flat cycle division, and then undergoes continuous division in all directions to form a mass of tissue. All of the above stages belong to the proembryo stage. Later, because the two sides of the top of this group of tissue split and grow faster, two protrusions are formed, which develop rapidly and become two cotyledons, and then gradually differentiate into embryos in the concave part between the cotyledons. At the same time, a cell at the top of the stalk below the globular embryo body, the hypophysis, and the basal cell of the globular embryo body continue to divide and grow, and differentiate into the radicle together. The part between the radicle and the cotyledons is the hypocotyl. Soon, due to the horizontal division of the cells, the cotyledons and hypocotyls are extended, and the hypocotyls and cotyledons are also bent into a horseshoe shape due to the limitation of space. At this point, a complete embryo body has been formed, and the embryo stalk degenerates and disappears.

development of monocot embryo

The embryonic development of monocots can be illustrated by the example of wheat in the Gramineae family. The development of wheat germ has something in common with the development of dicotyledonous plant embryo, but there are also differences. The first division of the zygote is oblique. It is divided into 2 cells, and then each of the 2 cells divides obliquely once to become a 4-cell proembryo. Afterwards, the four cells continued to divide from all directions, increasing the size of the embryo body. At the 16-32 cell stage, the embryo presents a club shape, with an enlarged upper part, which is the precursor of the embryo body, and a slender lower part, which differentiates into a stalk, and the entire embryo body is surrounded by a layer of protodermal cells.

When the embryo body of wheat has basically developed and formed, structurally it includes a scutellum (cotyledon), which is located on the inner side of the embryo and is close to the endosperm. The growth point at the top of the stem and the first true leaf primordium synthesize the germ, which is covered by the coleoptile. The end opposite to the germ is the radicle, which is covered by a radicle sheath. On the side opposite to the scutellum, the protrusions of the ectoderm can be seen. Some grasses, such as corn, do not have ectodermal leaves.

Endosperm development

Endosperm is the part of angiosperm seeds that store nutrients, and it develops after fertilization of two polar nuclei, so it is the product of triple fusion. After fertilization, the polar nucleus develops into the endosperm in the central cell without dormancy. The development of endosperm generally has three ways: nuclear type, non-cellular type, cellular type and helobial type. The karyotype is the most common, while the Helob-iales type is relatively rare, and only appears in the endosperm development of plants of the Helob-iales.

The development of the nuclear endosperm, the first division of the fertilized polar nucleus, and the subsequent period of nuclear division, without cell wall

Formation, each cell nucleus retains a free state and is distributed in the same cytoplasm, this period is called the formation period of free nuclei. The number of free nuclei often varies with plant species. As the number of nuclei increases, the nuclei and protoplasm are gradually squeezed to the surroundings of the embryo sac due to the appearance of the central vacuole, and are denser at the micropylar and chalazal ends of the embryo sac. On the side of the embryo sac, only a thin layer is distributed. Most of the nuclear divisions are mitotic, and a few appear amitotic, especially in the nuclei distributed at the chalazal end. The endosperm nuclear division proceeds to a certain stage, that is, the transition to the cell stage. At this time, the cell wall is formed between the free nuclei, and the cytoplasm is separated, that is, the endosperm cell is formed, and the whole tissue is called the endosperm. Monocots and most dicots fall into this category.

A whole genome duplication event in the ancestor of seed plants occurred about 319 million years ago. This gave rise to a series of evolutionary changes that resulted in the origin of modern seed plants.

A middle Devonian (385-million-year-old) precursor to seed plants from Belgium has been identified predating the earliest seed plants by about 20 million years. Runcaria, small and radially symmetrical, is an integumented megasporangium surrounded by a cupule. The megasporangium bears an unopened distal extension protruding above the mutlilobed integument. It is suspected that the extension was involved in anemophilous (wind) pollination. Runcaria sheds new light on the sequence of character acquisition leading to the seed. Runcaria has all of the qualities of seed plants except for a solid seed coat and a system to guide the pollen to the seed.

A propagule unique to gymnosperms and angiosperms, which is formed by pollination and fertilization of ovules. Seeds are generally composed of three parts: seed coat, embryo and endosperm. Some mature seeds of plants only have two parts: seed coat and embryo. The formation of seeds enables the young sporophyte jujube embryo to be protected by the mother and get sufficient nourishment like the fetus of mammals. Seeds also have various structures suitable for spreading or resisting unfavorable conditions, creating favorable conditions for the continuation of the plant race. Therefore, in the process of plant phylogeny, seed plants can replace ferns to obtain the dominant position. Seeds are closely related to human life. In addition to the grain, oil, and cotton necessary for daily life, some medicines (such as almonds), seasonings (such as pepper), and beverages (such as coffee and cocoa) all come from seeds.

Seed propagation of medicinal plants: Seed propagation of medicinal plants is the most common, such as ginseng , American ginseng, coptis, angelica, etc., which has the characteristics of simple propagation technology, large reproduction coefficient, and is conducive to introduction and domestication and cultivation of new varieties. However, the offspring of seed propagation are prone to variation, and flowering and fruiting are delayed, especially for woody medicinal plants, and the number of years required for seed propagation is also long.

Seed properties

The seed is a living organism in the dormancy period. The seed dormancy is limited by internal or external factors, and the phenomenon that it cannot germinate or germinates temporarily is a long-term adaptability of plants to external conditions. The phenomenon that the seeds cannot germinate temporarily because they have not passed the physiological post-ripening stage under suitable germination conditions after harvesting is called physiological dormancy; the phenomenon that the seeds cannot germinate temporarily because they cannot obtain the external conditions required for germination is called forced dormancy. The reasons for physiological dormancy are: firstly, the embryo is immature; secondly, although the embryo is fully developed in form, the stored material has not yet been transformed into a state that can be used by the embryonic development; thirdly, the differentiation of the embryo has been completed, but the protoplasm of the embryonic cell appears In the isolation phenomenon, there is a layer of lipid material outside the protoplasm, which reduces the permeability. Above-mentioned three kinds of situations all need the post-ripening action of seed itself to germinate. In addition, there are two situations: one is that there are substances that inhibit germination such as cyanic acid, nitrogen, plant alkaloids, organic acids, acetaldehyde, etc. in the fruit, seed coat or endosperm, which hinder the germination of the embryo; Too hard or waxy, poor water permeability and air permeability, affecting seed germination, seed dormancy is of great significance in production practice, plant hormones, and various physical and chemical methods can often be used to promote seed germination.

Seeds have a certain lifespan, and the lifespan of a seed refers to the vitality of the seed. That is, the longest lifespan that can be maintained under certain environmental conditions. The lifespan of various medicinal plant seeds varies greatly, and the short lifespan is only a few days or no more than one year. Seed life is directly related to storage conditions, and suitable storage conditions can prolong the life of seeds. However, fresh seeds are still used in production, because the germination rate of seeds in the next year is all reduced.