1901] DEVELOPMENT OF THE POLLEN 329 
This formation of microspores in a row, while not the usual 
tetrad form, is not peculiar to the Asclepiadaceae. The cells 
are reported in other arrangements in Typha latifolia,’ Orchis 
mascula,? Juncus,’*? and some members of the Periplocoideae.™ 
The cells now become isodiametric, irregular, and crowded. 
Adjusting themselves to each other they soon lose all indication 
of their origin as a row of four. The protoplasm becomes filled 
with short coarse threads which stain readily and obscure the 
other cytoplasmic structures (fig. zz). The walls then thicken 
very rapidly. This is by the deposition of a layer from within 
the cells, as is evident later when intercellular spaces appear. 
When a mature pollinium is cut and squeezed the walls separat- 
ing the microspores split, and the microspores are squeezed out 
whole. Where a microspore is in contact with a pollinium wall 
it separates from it an inner layer (fig. 76,4). This suggests 
that most of the pollinium wall is of a different origin from the 
walls of the microspores. The nucleus divides after the walls 
are thickened, forming the generative and tube nuclei (fig. 73). 
The tube nucleus is much the larger and denser. The generative 
nucleus moves to the side of the cell ( fig. 74), and is cut off by 
a wall of considerable thickness (fig. 75). The tapetum gradu- 
ally disintegrates, leaving the pollinium loose in the anther, 
except for its attachment to the caudicle by its upper end. 
After the walls begin to thicken the thready protoplasm collects 
in masses within the cells (jig. 12). The masses disappear 
about the time of the formation of the generative and tube 
nuclei, and dense fusiform bodies composed of one or more 
Strands, and of unknown nature, make their appearance. In 4, 
verticillata they were short, thick, and smaller than in the other 
species. Their origin may be connected with the thready and 
dense protoplasm just preceding their appearance. They 
8 SCHAFFNER, J. H., Development of the stamens and carpels of 7ypha /attfolia. 
Bor. Gaz. 24:93. 1897. 
°WILLE, Ueber die Entwickelungsgeschichte der Pollenkérner der Angio- 
spermen 39. 1886 
7 Ibid. 42. 
= SCHUMANN, Engler and Prantl, Nat. Pflanzenfam. q?: 196. 1895. 
