34 MORPHOLOGY OF ANGIOSPERMS 



A). The names used to designate these two layers indicate 

 their subsequent history, the former producing the wall of the 

 embedded sporangium, and the latter the sporogenous tissue. 



The cells of the primary parietal layer divide by periclinal 

 walls, so that usually a definite series of concentric parietal 

 layers appears (Fig. 8, A). Walls in other directions also ex- 

 tend the parietal layers uniformly with the rapidly enlarging 

 anther. The number of parietal layers is variable, but in most 

 cases there are from three to five. Sometimes there are only two 

 layers, as inSilphium (Merrell 2S ) and in Quercus (Conrad 29 ) ; 

 and among the Pontederiaceae Smith - 1 has regularly found six. 

 Even higher numbers have been reported, and Goebel 1X (p. 

 368) cites Agave americana as developing eight to twelve 

 fibrous or endothecial layers. In Rhopalocnemis phalloides 

 (Balanophoraceae) Lotsy 30 has shown that the sporangia of 

 the curious axial stamen do not organize definite parietal layers 

 and have no method of dehiscence, although the microspores 

 are fully matured. 



The outermost parietal layer usually develops very differ- 

 entlv from the others, and has been called the " endothecium." 

 This name was given by Purkinje 1 to designate all the layers 

 of the dehiscing anther w r all within the epidermis, which latter 

 he named the " exothecium." Since in most cases the outer- 

 most parietal layer is the only one represented in Purkinje's 

 " endothecium," the name has come to be restricted to it, which 

 seems to us unfortunate, for it should be retained in its original 

 application and used only in connection with the dehiscing 

 anther-wall. It remains true, however, that the outermost pa- 

 rietal layer generally becomes the endothecium, and in the fol- 

 lowing account this condition will be presented. If the anther 

 does not dehisce, the endothecial cells do not become specially 

 modified ; but if the anther dehisces, the cells develop thicken- 

 ing bands in various w T ays, the position and extent of these 

 banded cells being directly related to the method of dehiscence 

 (Fig. 7). 



Between the outermost and innermost parietal layers there 

 are usually one to three " middle layers," and this amount of 



applying the term to the first cell or group of cells differentiated from the 

 ordinary vegetative cells to produce sporogenous tissue, it is easy of applica- 

 tion and the homologies are definite. 



