66 MORPHOLOGY OP ANGIOSPERMS 



of Angiosperms the primary sporogenous cell is the mother-cell. 

 The only possible exception to this is the case of such sporog- 

 enous masses as occur in the ovules of Casuarina (Treub ), 

 Carpinus (Miss Benson 28 ), and Quercus (Conrad 53 ). If the 

 whole -sporogenous mass in these forms is derived from a hypo- 

 dermal archesporium, then of course the primary sporogenous 

 cells divide to form additional sporogenous cells. But if all 

 the sporogenous tissue is an archesporium, in this case con- 

 tributed to by cells deeper than the hypodermal layer, the pri- 

 mary sporogenous cells do not divide, nor do all the archesporial 

 cells give rise to parietal cells. In any event, the cells of the 

 completed sporogenous mass, whether archesporial or not, are 

 mother-cells. 



The history of the development of the microsporangia and 

 megasporangia is strikingly similar. In both cases the arche- 

 sporium is hypodermal ; in the microsporangium it is usually a 

 plate of cells and exceptionally a single cell, while in the mega- 

 sporangium it is usually a single cell and exceptionally a plate 

 of cells. In both each archesporial cell divides by a periclinal 

 wall, cutting off a peripheral parietal cell that takes part in 

 developing a sporangium wall of a variable number of layers. 

 In the development of the megasporangium, however, there is 

 a strong tendency to suppress the wall layers, probably as of 

 no significance or even a hindrance in the process of fertiliza- 

 tion. While in the microsporangium the primary sporogenous 

 cells often divide a few times before the mother-cell stage is 

 reached, this is by no means always the case; and although in 

 the megasporangium the primary sporogenous cells usually do 

 not divide to form mother-cells, this is probably not always true. 

 In both sporangia the mother-cells, reached by the same 

 sequence of events, are recognized by the fact that their division 

 is the reduction division. 



It is at this point that the history of the megasporangium 

 closes, for the reduction division is the beginning of the female 

 gametophyte (see p. 41). 



LITERATURE CITED 



1. Sc'iileiden, M. .T. Ueber Bildung des Eicliens mid Entstehung 

 des Embryo's bei den Phanerogamen. Nova Acta Leopoldina 

 19: 27-58. 1839; Ann. Sci. Nat. Bot. II. 11: 129-141. 1839. 



