Page:The Journal of Indian Botany, Volume III.djvu/56

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Mascre, M. Reeherches sur 1© developpement de Fanth&re chez les Solanacees (Contribution a l’etude de Fassize nourrici&re du pollen.) These Paris, 99 p,, 101 figs. 1921.

The author has investigated the origin, development, structure, and function of the tapetum in 20 species belonging to 14 genera of the Solanaceee. A section is devoted to a discussion of the merits of various fixing agents and staining methods for demonstrating the different cell structures, food materials, and phases of degeneration.

The structure and behaviour of the tapetum is quite uniform throughout the plants investigated. The hypodermal archesporial cells divide by peri- clinal walls to form the primary sporogenous cells and an outer primary parietal layer, which in turn divides to form the outer tapetum and a layer which gives rise to the wall cells of the anther. Early during this development, the vegetative cells bordering on the inner side of the sporogenous tissue differentiates to form the inner tapetum. Despite the fact that the tapetal cells have thus a dual origin, they all have the same structure ; this is because they are all subjected to the same physiological conditions, and all have the same function of transfering food materials to the developing pollen grains. The author entirely disagrees with the view sometimes expressed that the tapetal cells are potential sporogenous cells diverted for nutritive purposes.

The life of the tapetal cells is divided into three rather well-defined periods :

1. Period of differentiation, extending from the time the cells are first set apart, to tetrad formation in the pollen mother cells. The cells divide and attain their full number and size. The protoplasm is abundant, and there is a well-developed chondriome made up of granules and short rods. The cells are always uninucleate at first, but the nuclei soon begin to divide to produce two .or four daughter nuclei, which then show a decided ten- dency to fuse into large polyvalent nuclei.

2. Stage of maturity, during which the tapetum is actively functioning as a secreting layer. The chondriome transforms into long rods, some of which give rise to plastids in which starch grains are formed, while others give rise to granules of fatty substance. These food substances are gradually passed inward in solution to the developing pollen grains. The nuclei finally complete their fusions, and some of the large fusion nuclei may again undergo mitosis.

3. Period of degeneration. This stage begins about when the walls of the pollen grains commence to thicken. The first sign of degeneration is seen in the chondriome ; then the tapetum walls become cutinised. At length the nuclei and the cytoplasm with its chondriome degenerate, and at the time of anther dehiscence only the shriveled remnants of the tapetum cells remain. Here and there cells or call contents are found wandering out