36 



MORPHOLOGY OF ANGIOSPERMS 



variation may occur in the same Avail, as noted by Coulter 1!> 

 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 



D 



Fig. 11. — Zostera marina. A, young mierosporangium with archesporial cells shaded. 

 -ff, later stage showing tapetum derived from sporogenous cells ; t, 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. Z>, portion 

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

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

 .Rosenberg. 32 



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 



