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Reproductive structure and development of angiosperms (Illustrated)

Angiosperms are the largest and most advanced species in the plant kingdom, with about 300-450 families and genera in the world. The embryo sac composed of a few (typically 8) cells and the phenomenon of double fertilization are regarded as evidence of the evolutionary consistency of angiosperms and the difference from other plant groups. This article mainly introduces the frontal reproductive structure and development process of angiosperms.

Reproductive development of angiosperms

Reproductive structure and development of angiosperms

1、 The structure and function of male reproductive organs in angiosperms are as follows

1. Anther structure;

There is usually only one layer of cells in the inner wall of the anther chamber, and a large amount of substances are stored in the initial stage. When anther matures, the cells expand radially and the storage materials disappear. Except the tangential wall, there are many oblique longitudinal stripes of cellulose and secondary thickening, so it is called fibrous layer. When the inner wall of anther chamber forms fibrous layer, a long and narrow parenchyma cell gap is often left at the junction of two adjacent pollen sacs.

The middle layer is composed of one to several layers of smaller cells, which can store starch and other nutrients at the initial stage, and then the cells are squeezed and gradually disintegrated and absorbed, so they have disappeared in the mature anther.

The tapetal cells are large and mononuclear at the beginning, but binucleate or multinuclear at the beginning of meiosis. It is rich in protein and protein, and plays an important role in regulating the development of degenerated organelles. Tapetum can synthesize and secrete callase, decompose callose wall of pollen mother cell and tetrad, and separate mononuclear pollen grains. The tapetum also synthesizes proteins, which transport to the outer wall of pollen grains, which is a recognition protein.

After the pollen grains mature, the cells in the fibrous layer lose water, and the mechanical force makes the anther break at the crack, and the pollen grains disperse from the crack formed by the longitudinal axis of the crack. The wall of the pollen sac disappeared due to the disintegration of the tapetum, or there were only traces, only epidermis and fibrous layer.

The structure of anther (Illustrated)

2. Anther development;

Anthers consist of pollen sacs and connectives. At the early stage of anther development, the structure is simple, the outer layer is a layer of proepidermis, the inner layer is a group of basic meristem; soon, because the four corners of anther split quickly, the anther is tetragonal. After that, many rows of sporogonia with large volume, large nucleus, dense cytoplasm, long radial wall and strong division ability were differentiated under the proepidermis at the four edges. The outer layer is the primary peripheral layer; the inner layer is the primary sporogenetic cell, and the primary peripheral layer cells continue to divide horizontally and vertically, and gradually form the inner wall, middle layer and tapetum of the anther chamber. The cells in the middle part of anther gradually divide and differentiate into vascular bundles and parenchyma cells, which constitute the connective.

3. Production of microspore;

The sporogenous cells divide several times and form pollen mother cells. Pollen mother cells are closely arranged and have a general cellulose wall in the early stage. In the whole pollen sac, there are plasmodesmata between pollen mother cells and tapetal cells. Between the adjacent pollen mother cells, cytoplasmic tubes with a diameter of 1-2 microns are often formed, which connect the pollen mother cells of the same pollen sac into syncytia. It was found that organelles such as chromatin and endoplasmic reticulum fragments and nutrients communicated through cytoplasmic tubes. This phenomenon is related to the synchronization of meiosis of pollen mother cells in pollen sacs and the rapid transportation and distribution of nutrients and growth substances.

Later, during meiosis, as pollen mother cells accumulate callose between plasma membrane and cell wall, callose wall forms and thickens gradually, resulting in the blocking of plasmodesmata and cytoplasmic tubules. Before meiosis, the morphology and structure of pollen mother cells often have many changes, such as the increase of cell and nucleus volume, the thickening of cytoplasm, the synthesis of RNA and protein, the density of nucleosomes, the increase of plastids and mitochondria, the small but not obvious vacuoles, etc.

4. The development of pollen (male gametophyte);

Primary SPOROGENIC cell -- pollen mother cell -- tetrad -- single cell pollen grain -- two cell pollen (vegetative cell, germ cell) -- three cell pollen (vegetative cell 1, sperm 3)

5. Structure and function of mature pollen grains

Pollen wall, vegetative cell, germ cell and sperm.

Mature pollen grains and honeybees

2、 The structure and development of female reproductive organs

1. Structure of pistil;

The pistil is composed of carpels. The carpels have three vascular bundles. The dorsal bundle is equivalent to the leaf vein, and the ventral bundle is on both sides. The carpel is rolled inward to close the adaxial side (or ventral side). The junction of carpel edge is called ventral suture, and the dorsal bundle is called dorsal suture. Single pistil has only one ventral suture and one dorsal suture. In compound pistil or syncarp pistil, the number of ventral and dorsal sutures is the same as that of carpel. After the carpel is rolled into pistil, the upper end is stigma, the middle is style, and the lower end is ovary.

Pistil structure (diagram)

2. The structure of mature ovule;

Chalazal point: the base of the nucellus, the place where the integument, nucellus and petiole join.

Nucellus: the most important part of an ovule, the parenchyma in the integument, in which megaspores are produced and further developed into an embryo sac.

Integument: the structure that envelops the nucellus. Some plants may have outer integument and inner integument.

Micropyle: a small hole left by an unclosed front end of the integument.

Petiole: the petiole of the ovule at the base of the placenta, through which vascular bundles enter the ovule.

Ovule structure type (diagram)

3. Ovule development;

During the development of ovule, some cells in the lower epidermis of placenta divide horizontally and produce a protuberance, which becomes the primordium of ovule. The anterior end of the primordium becomes the nucellus. The nucellus is a group of similar parenchyma cells, and the base of the primordium becomes the petiole. Because the epidermal cells at the base of the nucellus divide rapidly, produce a ring-shaped protrusion, grow and expand upward gradually, and surround the nucellus. The tissue surrounding the nucellus is the integument, leaving only a small hole in the front of the nucellus, that is, the micropyle.

The development of ovule (Figure) 4. The structure and development of embryo sac;

Embryo sac is the female gametophyte of angiosperms, which is composed of egg cell, helper cell, polar nucleus and antipodal cell.

The egg cell is a highly polar cell. Its wall is usually the thickest at the micropyle end, and the wall near the chalazal end gradually becomes thinner. The nucleus of egg is large and the RNA content of nucleolus is higher than that of other cells in embryo sac. In mature oocytes, plastids and mitochondria often degenerate and decrease in number, endoplasmic reticulum and Golgi apparatus often become rare or underdeveloped, reflecting the low intensity of metabolic activity of oocytes.

The synergid and egg cells are arranged in a triangle at the end of the micropyle. They are also highly polar cells. The wall of the synergid, like that of the egg cell, is thickest at the micropylar end and thins gradually towards the chalazal end. The most prominent feature of the synergid is that there is a filamentous organ on the cell wall of the micropyle end, which is the part of the wall extending inward, similar to the pleated process of the transfer cell wall. The composition of different plants varies. The structure of filamentous apparatus greatly increases the surface area of plasma membrane, which may be related to the function of synergid.

The structure of embryo sac (Illustrated)

The central cell is the largest and highly vacuolated cell in the embryo sac. The enlargement of mature embryo sac is mainly due to the enlargement of central vacuole. The wall thickness of the central cell varies greatly. At the junction with the egg cell and the central cell, there is usually only plasma membrane but no cell wall; at the junction with the antipodal cell, there is a thin wall of plasmodesmata. There are also many finger like protrusions on the inner side of the central cell wall in many plants, indicating that it can absorb nutrients from nucellar tissue or integument tissue. Abundant plastids, nucleosomes, mitochondria, Golgi apparatus and endoplasmic reticulum are found in the cytoplasm of the central cell in many plants. The polar nucleus or secondary nucleus of the central cell is often suspended in the middle of highly vacuolized cells by many cell cords. The polar nucleus is usually large, and the nucleolus is also large.

Antipodal cells are the most variable cells in the embryo sac. Not only the number of cells varies greatly, but also the structure of cells varies with plants. Antipodal cells have the function of absorbing nutrients from the nucellus and transferring them into the central cell through antipodal cells. The cytoplasm of antipodal cells in most plants is rich in plastids, nucleosomes, mitochondria, Golgi apparatus and endoplasmic reticulum. Antipodal cells are very active in metabolism. They can absorb, transport and secrete nutrients during the development of embryo sac. In most plants, antipodal cells are usually short-lived and degenerate before or shortly after fertilization, except that antipodal cells of some plants can exist for a long time.

3、 Development of embryo sac:

First of all, some cells in the lower layer of placenta epidermis undergo pericyclic division and produce a mass of protuberances to become ovule primordia. The anterior end of the primordium becomes the nucellus. The nucellus is a mass of similar parenchyma cells. The base of the primordium becomes the funicle.

Under the epidermis near the end of the micropyle, a different cell, namely sporogonia, is gradually formed. The sporogonia are large in size, dense in cytoplasm, rich in organelles, high in RNA and protein, low in vacuolation, large and prominent in nucleus, and many plasmodesmata on the wall. At the time of differentiation, the nuclear DNA was synthesized to tetraploid or 4C level. After the formation of sporogonia, they need to further develop into embryo sac mother cells. The form of its development varies with different plants. In many angiosperms, including cotton and other crops, the sporogonia first undergo a pericyclic division to form two inner and outer cells: the outer one is called peripheral cell, and the inner one is called SPOROGENIC cell. The peripheral cells continued to undergo pericyclic and anticlinal division, increasing the cell layers of nucellus, while the sporogenetic cells grew to form embryo sac mother cells.

The first division of embryo sac mother cells in meiosis: because the epidermal cells at the base of nucellus divide rapidly, they produce a ring-shaped protrusion, grow and expand upward gradually, surround the nucellus, leaving only a hole at the front end of the nucellus. The tissue surrounding the nucellus is the integument.

The second division of meiosis: the formation of tetrads. Three cells near the micropyle degenerated. The cells near the chalazal end gradually developed into mononuclear embryo sac. In addition, the nucellar nucleus was slightly enlarged and large vacuoles appeared. Later, the mononuclear embryo sac underwent three mitoses to form an octanuclear embryo sac.

In the first division, they moved to the two ends of the embryo sac cells to form a dinuclear embryo sac. The second division forms a tetranuclear embryo sac. The embryo sac cells grew further and the nucleus temporarily dissociated from the common cytoplasm. The third division forms an octanuclear embryo sac. Then, one of the four nuclei at each end moved to the middle of the embryo sac and moved closer to each other.

In mature octanuclear embryo sac, a small hole, or micropyle, is usually left at the front of integument.

Mature octanuclear embryo sac (pictured)