. Morphology of angiosperms (Morphology of spermatophytes. Part II). Angiosperms; Plant morphology. THE MICROSPORANOIUM. Fig. 12.—Lemna minor. Section of mierospo- rangium showing some of the spore mother- cells broken down and functioning as tape- turn ; x 1100.—After ' layer of cells is the rule, the tapetum may include two or more layers, as pointed out by Frye 33 in Asclepias. The same inves- tigator has also followed the origin of that portion of the tape- turn next the connective from the plate of cells im- mediately within the arche- sporium; and in a recent paper Rosenberg32
. Morphology of angiosperms (Morphology of spermatophytes. Part II). Angiosperms; Plant morphology. THE MICROSPORANOIUM. Fig. 12.—Lemna minor. Section of mierospo- rangium showing some of the spore mother- cells broken down and functioning as tape- turn ; x 1100.—After ' layer of cells is the rule, the tapetum may include two or more layers, as pointed out by Frye 33 in Asclepias. The same inves- tigator has also followed the origin of that portion of the tape- turn next the connective from the plate of cells im- mediately within the arche- sporium; and in a recent paper Rosenberg32 de- Eosenberg ; scribes and figures the much elongated archespo- rial cells of Zostera as cut- ting off isodiametric cells at each end, that divide more or less and form the tapetum on the outer and inner surfaces of the spo- rogenous mass (Fig. 11, B, t). There is evidence, therefore, that in certain cases the tapetum, or at least part of it, may be derived from sterile cells cut off from the periphery of the sporog- enous mass. Such a probability is also reported bv Coul- ter 10 in Ranunculus, and by Webb34 in Astilbe. Enough is known, at least, to lead to the conclusion that any sterile cells in contact with the sporogenous tissue assume the tape- tal function. This is a well-known fact in connection with sterile mother-cells, which in this sense are a part of the tape- tum. Among the Pontederiaceae Smith21 found that the tapetal cells, closely adherent to the mother-cells, are often wedged among them; and in Lemna Caldwell 23 observed that the cells of the regular tapetal jacket often divide and form groups of cells projecting deep among the mother-cells, sterile mother-cells also disintegrating (Fig. 12) ; while in Symplo- carpus Duggar 27 reports that the tapetal cells become free and " wander " among the mother-cells. It seems clear, therefore, that the tapetum is a set of sterile cells that nourish the func- tioning mother-cells, and that while ordinarily
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