THE MICROSPORANGIUM 



37 



FIG. 12. Lemna minor. Section of microspo- 

 rangium showing some of the spore mother- 

 cells broken down and functioning as tape- 

 turn : x 1100. After CALDWELL. SS 



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- 

 tum next the connective 

 from the plate of cells im- 

 mediately within the arche- 

 sporium; and in a recent 

 paper Rosenberg 32 de- 

 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 by Coul- 

 ter 19 in Ranunculus, and by Webb 34 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 Smith 21 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 it is a definite 

 layer none of which is derived from the primary sporogenous 

 cells, it may include a variety of morphological elements. 

 As a rule, the complete organization of the tapetal jacket is- 



