90 DuGGAR : Development of the Pollen Grain 



before the latter stage has resulted, the outer layer of periblem in 

 general has become well differentiated, some of the hypodermal 

 cells in the region m, in have already lost their previous slight 

 differential character (or lose it entirely with further development), 

 and the four regions of Fig. 2 are evident, but not sharply defined. 

 Thus it would seem difficult here to locate the sporangia relative 

 to the surface of the sporophyll on which borne, unless beginning 

 with such a stage as Fig. 2, where the connective, m, is already 

 distinctly different from the remaining portions of the hypodermal 

 layer, and might be taken as separating an upper from a lower 

 surface. Engler's * studies on many forms led to the conclusion 

 that in both extrorse and introrse anthers two sporangia are borne 

 on each surface. My observations on Bigiionia suggested at an 

 earlier stage a common fundament for each pair of sporangia in 

 radial arrangement, and the improbability of any distinction between 

 upper and lower surfaces in the young condition. Moreover, from 

 what we know of stamens which have been partially changed from 

 staminal to purely floral organs, as In Caniia and in the pond lily, 

 there is no such location of sporangia, it would seem. 



The cells of the hypodermal layer at x, x andjF, J', Fig. I, di- 

 vide by periclinal walls into two layers, as in Fig. 5. The Inner of 

 these layers rapidly becomes rich in protoplasm, the nucleus 

 increases in size and the cell wall in thickness, and there results 

 the primitive archesporlum of about six or eight cells in each spor- 

 anglal region, in cross section. In the primitive archesporlum the 

 cells undergo no further periclinal divisions, and a single layer is 

 maintained until maturity ; but at a later period, when the cells 

 have increased in size and peculiarity, a few radial divisions occur, 

 as in Figs. 6 and 10, usually increasing the extent of the layer to 



about thirteen cells at the middle part 



' In the meantime the cells of the wall layer (secondary hypo- 

 dermal) divide, forming on the inside a layer of cells soon differ- 

 entiated as the outer tapetum, and on the outside the first true wall 

 layer, Figs. 5 and 6. The next periclinal division throughout the 

 wall layer, Fig. 7, completes the general development on the outer 



^En^Ver, A., Beitriige zur Kenntni.ss der Anthercnhildnng der Mclaspermen. 

 Jahrb. wlss. But. lo : 275-316. 1876. Compare IV., " Uber die sogenannten in- 

 trorsen und extrorsen Antheren, pp. 299-305.'' 



