THE MICROSPORANGIUM 33 



of hypodermal cells becomes differentiated in each lobe, dis- 

 tinguished from the adjacent cells by their larger size, their 

 usual radial elongation, their larger nuclei, and their different 

 reaction to stains. In cross-section this plate is a single row of 

 cells of variable number, sometimes almost equaling in extent 

 the contour of the lobe, as in Mentha aquatica (Warming 8 ) ; 

 sometimes consisting of four to six cells, as in Orchis maculata 

 (Guignard 10 ) ; sometimes three or four cells, as in Iiemerocal- 

 lis fulva (Fullmer 24 ) ; sometimes one or two cells, as in Conval- 

 laria majalis and Potamogeton foliosus (Wiegand 25 ) ; and 

 sometimes constantly one cell, as long known in Malvaceae and 

 most Compositae, and recently reported in Avena fatua by Gan- 

 non. 26 In longitudinal section the plate extends approxi- 

 mately the length of the anther, being a single row of cells 

 in case the cross-section consists of a single cell ; but in Mimo- 

 seae the whole archesporium is reported by Rosanoff 4 as being a 

 single cell, as is also the case in Euphorbia corollata, as re- 

 ported by Miss Lyon. 22 The general fact becomes clear, there- 

 fore, that an exceedingly variable amount of the hypodermal ! 

 layer may become archesporium, from nearly all of it to a single 

 cell ; and further, that this amount usually varies within cer- 

 tain limits ,in the same species, and that the extent of the 

 archesporium is in no way related to the primitive or highly 

 specialized character of plant groups. 



The subsequent divisions to the mother-cell stage usually 

 follow one another rapidly (Fig. 10). Following the history 

 of a single sporangium, the radially elongated archesporial cells 

 all divide equally and almost simultaneously by periclinal walls, 

 forming an outer layer (primary parietal,* Fig. 9, A, b) and 

 an inner layer (primary sporogenous,f Figs. 9, A, a, and 10, 



* This has been commonly called the "primary tapetal layer," on the 

 ground that the tapetum is one of its derivatives. At most only a part of the 

 tapetum can be derived from it, and in some eases none of the tapetum is so 

 derived. Besides, the tapetum is a physiological layer of variable morpho- 

 logical origin. The essential morphological feature of this outer sterile layer 

 is that it develops the wall of the embedded sporangium, and hence we have 

 preferred to designate it as the primary parietal layer. 



f This is the "archesporium" of Goebel's Outlines of Classification and of 

 other texts. With such an application of the term the archesporium of the 

 mierosporangium of Angiosperms does not homologize with that of the mega- 

 sporangium, and is of indefinite application among the Pteridophytes. By 



