36 



MORPHOLOGY OF ANGIOSPERMS 



variation may occur in the same wall, as noted by Coulter 19 

 in Ranunculus. The cells of these layers are usually tabular, 

 and gradually become flattened and disorganized; but in some 

 cases the one or two innermost middle layers become prominent 

 as a part of the tapetum ; in others the outer ones may become 

 a part of the endothecium; and occasionally there is no dis- 

 organization of parietal layers. 



The innermost parietal layer, as a rule, becomes part of the 

 tapetum, which is a jacket of nourishing cells in immediate con- 

 tact with the sporogenous tissue (Figs. 9, 10). The tapetum has 



FIG. IV. Zostera marina. A, young microsporangium with archesporial cells shaded. 

 B, later stage showing tapetum derived from sporogenous cells ; tf, tapetum ; p, pollen 

 mother-cells ; st, sterile cells, as shown by transverse wall. C, portions of the two 

 long cells resulting from the first division of the pollen mother-cell. D. portion 

 of a microspore showing the nuclear division that gives rise to the generative 

 and tube nuclei ; there are six chromosomes. E, the filiform pollen grain. After 



KOSENBERG. 83 



no definite morphological boundary or origin, but results from 

 pressing into special physiological service the sterile cells, of 

 whatever origin, contiguous to the sporogenous tissue. While one 



