. Botany of the living plant. Botany; Plants. THE CARPEL AND OVULE 297 upgrowth : it consists internally of a number of radial rows of cells, covered by a superficial layer. The latter divides in Caltha into two or more layers at the tip of the nucellus, forming a cap of tissue covering the radial rows within. It is from the central row of the internal cells that the embryo-sac arises. The condition in Caltha is relatively simple. The terminal cell of the central row undergoes division into two, and then into four (Fig. 217). This is in fact a tetrad-division, and the mother-cell which divides
. Botany of the living plant. Botany; Plants. THE CARPEL AND OVULE 297 upgrowth : it consists internally of a number of radial rows of cells, covered by a superficial layer. The latter divides in Caltha into two or more layers at the tip of the nucellus, forming a cap of tissue covering the radial rows within. It is from the central row of the internal cells that the embryo-sac arises. The condition in Caltha is relatively simple. The terminal cell of the central row undergoes division into two, and then into four (Fig. 217). This is in fact a tetrad-division, and the mother-cell which divides is of hypodermal origin. It has been shown that this division is accompanied by reduction of chromosomes of the nuclei to the half number, as in the pollen-tetrad. The resulting four cells are arranged in a row ; pollen-tetrads are sometimes found to have the same arrangement. The conclusion follows that the tetrad thus produced in the ovale is the correlative of a single pollen-tetrad, and each of the four cells might become a spore. This actually happens in the pollen-sac ; but in the ovule as a rule only one of the four potential spores develops further. The lowest cell of the four enlarges at the expense of the others, which collapse, and are crushed out of shape. The embryo-sac encroaches also on the surrounding cells of the nucellus, which give way to allow of its increase in size. Thus, as shown by its development, the embryo- sac is the single spore of a tetrad : as it develops to a large size it is styled a mega-spore. In other cases the structure may be more complex than in Caltha. A considerable number of Flowering Plants show rapid growth and division of the superficial cells at the tip of the nucellus, so as to form a considerable pad of tissue covering the hypoderma. This is seen in Rosa livida (Fig. 218), which also shows numerous hypo- dermal cells with dense contents, each divided into a cell-row. The basal cell of each row is an embryo-sac mother-cell ; the d
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