Pollen grains develop in the anthers of the staminae. In the anthers (figure A and detailed view D) mostly four, but sometimes only two loculi are present. In the loculi sporogenic tissue (from the Greek spora = seed and the Latin generare = produce) can be found from which pollen develop. At the inner side of each loculus a layer of large, rectangular cells, the tapetum (from the Greek tapes = carpet; t in figure D) can be found. The tapetum serves for the nutrition of the developing pollen, the deposition of cell wall material in the outer part of the pollen grain and other compounds in and over the wall. First, free pollenmothercells (PMC) are formed, which become spores by a meiotic division (see example of a meiotic stage in figure B). The meiosis involves two divisions, which lead to the formation of four daughter cells, the spores. Those four cells are originally still interconnected and are called tetrads (Greek Tetra = four; figure C). Later they come apart and the tapetum deposits the outer wall or exine (more on the pollen wall in pollen morphology). The exine protects the spore against dessication, mechanical pressure and ultraviolet radiation (figure F). Sometimes the exine layer is covered by sticky substances (pollenkitt, tryphine, elastoviscin and sporopollenin viscin threads), which are also produced by the tapetum. This adhesive material facilitates the attachment of pollen grains to insects, and in this way also zoophilic pollination. It also plays an important role in the adhesion of pollen grains to the female stigma and in the recognition between pollen and pistil. Also substances responsible for pollen allergy are often products originating from the tapetum.
Pollen development in the anthers
A. Cross-section through an anther of Lilie (Lilium)sp.) with on the left and the right side two loculi each. In the loculi sporemothercells (SMCs) can be seen from which the four spores develop through meiosis I and II. Inbetween the loculi of each pair a thin layer of cells (arrow) is visible along which the loculus can burst open atmaturity and release the pollen grains. In the middle the cross-sectioned filament (Fi) to which the anther is attached is indicated. In the upper part the vascular bundle (v) of the loculus can be distinguished.
B. Loculus. The lumen contains developing pollen. On the inner wall (w) of the loculus a layer constitued of block-shaped single cells is present, the tapetum (t). The tapetum feeds the developing spore and -later- pollen.
C. Tetrad stage during pollen development. After the two meiotic divisions the four daughter cells are still interconnected and form a tetrad. They are still surrounded by the wall (arrow) of the original cell, the microspore mothercell (MMC).
D. Mitotic division in the spore leading to the formation of a microgametophyte or pollen. Only the metaphase is shown here. The chromosomes lay in the equatorial plane of the cell.
E. Nearly ripe pollen grain: visible are a vegetative cell with nucleus (VN), which later will form the pollen tube, and a generative cell with its own nucleus (GN), which later will divide into two sperm cells.
F. Ripe pollen grain in which the texture of the outer cell wall, the exine, can be recognized. The grainy dark purple structure in the middle of the pollen grain is the vegetative nucleus.
G. Diagram in 3 parts: Ripe pollen grain consisting of the vegetative cell (VC) and therein the smaller generative cell (GC). After landing on the stigma (St) the pollen grain germinates and forms a pollen tube. In the pollen tube the generative cell divides into two sperm cells (SC). The pollen tube grows to the embryo sac (ES) and delivers the two sperm cells that are involved in double fertilization.
Mature pollen grain and pollen tube in flowering plants
In the final ripening phase of the spore a division occurs, giving rise to two cells. The spore has now become a bi-cellular, male gametophyte or pollen grain.
Pollen grain and pollen tube
Cells, germination and growth
A. Ripe pollen grain and B. pollen tube in tobacco
Only the nuclei, which have been stained with the fluorescing probe DAPI, are clearly visible here:
The generative nucleus = NK as part of the generative cell,
The vegetative nucleus = VN, part of the vegetative cell
The two sperm nuclei = SN, belonging to the two sperm cells
Germination in vitro of colorata pollen 64x speeded up; Source: YouTube
Germinating pollen grain and pollen tube in Galanthus nivalis A + B Pollen germination, C Growing pollen tube and D Generative cell (GC) in a pollen tube Source: Film by dr. I. Lichtscheidl c.s. from the department for Cell Physiology and Scientific Film of the university of Vienna
Most striking is the large cell, the vegetative cell (VC in figure G hereabove) with many organelles and an own nucleus, the vegetative nucleus (VN in E hereabove and left). The vegetative cell is surrounded by a thick, strongly sculptured cell wall (see Pollen morphology). This large cell encloses a second, much smaller, spindle-shaped cell, which is called generative cell (GC in G hereabove) and which also bears an own nucleus, the generative nucleus (GN in E hereabove and left). After maturation the pollen grains enter a phase of quiescence and dry out together with the anthers. When pollen grains reach a mature stigma (the upper part of the pistil; St in F hereabove) of the same species following pollination by wind, animals or water, they take up water. As a result the rehydrated pollen grains can reactivate and start to germinate and form a tubular extension, the pollen tube. The generative cell will divide once more to produce two sperm cells (SC in F hereabove and left; Note: plants exhibit a double fertilization); these sperm cells are thus the actual male sex cells or gametes. The division of the generative cell into a sperm cell can take place at various stages of development, in some species this division is already concluded in the anthers, in other plants it takes place in the pollen tube, shortly after the germination of the pollen grain.
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