No. 472] 



POLLEN GRAIN VARIATION 



277 



A comparison between angiosperms and gymnosperras as re- 

 gards the structure of the pollen grain recalls the well known 

 difference. The essential structure of the pollen is much more 

 uniform among the angiosperms, showing a spermatogenous cell 

 usually free within the larger tube-forming cell, the whole structure 

 to be regarded as probably a gametophyte reduced to an antheri- 

 dium, the tube cell forming the antheridium wall, and generally 

 there are no prothallial cells. Among the gymnosperms the struc- 

 ture of the pollen grain is not uniform in the different families and 

 genera, and it is not so much reduced as in the angiosperms. In 

 the more complex of the gymnosperms (Picea for example) there 

 is not only an antheridium represented, but there may be from 

 one to three disintegrating prothallial cells, and in addition the 

 antheridium has a cell, the so called stalk cell, which has nothing 

 to correspond to it in the angiosperms. This stalk cell is a sister 

 cell to the so called body cell or central cell, which divides to form 

 (usually) two sperm cells, and therefore corresponds to the sperm- 

 atogenous cell in the angiosperms. In those g\'mnosperms with 

 the simpler pollen grains no disintegrating prothallial cells are 

 formed, but the stalk and central cells are uniformly present, the 

 mother cell of these two being cut off at one side of the large tube- 

 forming cell. In the pollen of the simplest gymnosperms there- 

 fore theiv is one cell more than in that of the angiosperms, that is 

 the stalk cell. ( "r\ |)t<)ineria seems nearer to the angiosperms in 

 this particular than any other gymnosperm, since Lawson (:04) 

 has found that the cell cut off at the side of the pollen grain is 

 soon set free and lies within the cytoplasm of the large tube-forming 

 cell. Chamberlain ('97) has also found cases in pollen grains of 

 angiosperms which show one prothallial cell and more often it is 

 found in angiosperm pollen that the spermatogenous cell is cut 

 off at one side of the grain instead of lying free in the tube cell 

 (Coulter and Chamberlain. : 03, pp. 13-4-135). 



