. Botany for agricultural students. Plants. STAMEN 461 Stamen. — The stamen (microsporophyll) has its pollen sacs (microsponingia), usually four in number, joined into the struc- ture called anther. The pollen grains (microspores) are numer- ous in each sac and are formed before the flower opens: Like the spores of Gymnosperms, Pteridophytes, and Bryophytes, they are formed by special cells known as mother cells of which there are many in each pollen sac as shown at A in Figure 408. These mother cells also divide by the reduction division, that is, by the kind of cell division in which the dau
. Botany for agricultural students. Plants. STAMEN 461 Stamen. — The stamen (microsporophyll) has its pollen sacs (microsponingia), usually four in number, joined into the struc- ture called anther. The pollen grains (microspores) are numer- ous in each sac and are formed before the flower opens: Like the spores of Gymnosperms, Pteridophytes, and Bryophytes, they are formed by special cells known as mother cells of which there are many in each pollen sac as shown at A in Figure 408. These mother cells also divide by the reduction division, that is, by the kind of cell division in which the daughter nuclei get only half the sporophytic number of chromosomes. The mother cells. A B Fig. 408. — The spore mother cells of Angiosperms. A, cross section of a young anther, showing the microspore mother cells (//i). B, section through an ovule, showing the megaspore mother cell (?»). Both are highly magnified. are formed and undergo the reduction division while the flowers are still small buds. Immediately following the division of the mother cell, the daughter nuclei resulting from this division divide and consequently there are four spores or pollen grains formed from each mother cell. The four spores constituting the progeny of a mother cell are called a tetrad. The cells of the tetrad commonly cling together for a short time after they are formed, but soon separate and each becomes a pollen grain. The pollen grains are in reality the one-celled stages of the male gametophytes, since they have the reduced or gametophytic number of chromosomes. Usually before the pollen grain leaves the anther its nucleus divides, forming a tube and genera- tive nucleus. In this condition the pollen grain is carried to the. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Martin, John N. (John Nathan), b. 1875. New Y
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectplants, bookyear1919
