390 Cooke and Schively on Observations on the 



Development of the Pollen Grain. 



In the anthers of the very young cleistogamic buds, the 

 following stages of microspore development were noted. The 

 anther wall shows two or three rows of embryonic cell tis- 

 sue, with unthickened walls. Within, the sporangial cavi- 

 ties are completely filled by regular rows of large, nearly 

 cubical "spore-mother" cells. They are filled with cyto- 

 plasm that reveals a distinct network structure of stained 

 nodes and pale connecting threads. Each cell has a large 

 nucleus, and its clear cavity is scantily filled with granules 

 of chromatin lying on slender linin threads. There is a 

 large spherical pale nucleolus that appears entirely unstained 

 (Fig. 17a). This nucleolus is strikingly different from any 

 other observed throughout the entire plant. The macro- 

 spore cells, on the other hand, show the deepest stained 

 nucleoli present in the plant. 



Later, this mother-spore nucleus goes into a distinct syn- 

 apsis state (Figs. 17 b-d). The nucleolus grows even 

 fainter, and the chromatin gathers about it, passing in along 

 the linin threads that are seen streaming from all parts of 

 the cytoplasm to the nucleolus. Finally, the nucleolus is 

 entirely covered up by the great crowded, densely staining, 

 balled-up chromatin mass, while the nuclear cavity around 

 is perfectly clear-looking and unstained. It is a typical syn- 

 apsis stage. Some time later this nucleus goes into a typical 

 resting stage, and then shows chromatin granules on a 

 fine network of linin threads, in a clear nuclear cavity. The 

 nucleolus has entirely disappeared (Fig. 17 e). Finally, the 

 spore mother cell undergoes its first division. A broad, 

 barrel-shaped spindle is formed, very unlike the slender 

 pointed spindles of the macrospore divisions (Fig. 17 f). 

 Then two large nuclei lie side by side in the spore proto- 

 plasm (Fig. 17 g). These divide again, and the four smaller 

 nuclei lie close together in the space, their contact walls 



