Mar. II, 1918 Sterility in the Strawberry 653 



Although the gametophytic generation must properly be considered 

 as beginning with the first appearance of the haploid chromosome 

 number, yet the liberation of the spores from the tetrad may be con- 

 sidered as marking the beginning of the independent growth period of 

 this generation and the rapid growth of the spore wall, division of the 

 microspore nucleus, the increase in cytoplasm, and finally the germi- 

 nation of the pollen grain and the production of sperms as develop- 

 mental stages in this very much foreshortened plant generation. The very 

 rapid wall growth, division of the nucleus, and finally the increase in 

 cytoplasm to 7.5 times its original volume, all within a relatively short 

 period, go to make this period of a plant's life history probably the 

 most finely adjusted and critical one which it is called upon to survive. 



It is during the periods of rapid enlargement and of increasing cyto- 

 plasm that degeneration of the microspores takes place. 



Although there is some evidence in the strawberry that degeneration 

 may begin before liberation of the microspores from the tetrad, so many 

 anthers have been examined which contain only normal microspores, 

 both in the tetrad stage and directly following their liberation, that 

 degeneration before this time can be considered as negligible. It is 

 probable that poor fixation may account for the few apparent instances 

 which have been found. Indeed, poor fixation constantly enters as a 

 disturbing factor in the interpretation of the condition of the supposedly 

 aborting grains, and it is only by finding the same types at later and 

 more advanced stages of degeneration when there can be no question as 

 to the condition of the microspore that the factor of poor fixation can 

 be eliminated. No aborted microspores have so far been found which 

 could be referred to as degeneration within the tetrad. 



Directly following microspore liberation evidences of pollen abortion 

 may be noticed and from this period on through the various stages of de- 

 velopment, microspores and pollen grains are continually aborting. 



Plate D, figures 3 and 4, shows microspores shortly after liberation, in 

 which the contents have completely broken down into a yellow oily mass 

 which turns black on exposure to osmic acid. The lighter areas are 

 globules evidently of a different substance. 



Ordinarily during the growth period of the wall following liberation, 

 the cytoplasm becomes spread over the periphery in a thin layer. Plate 

 D, figures 5, 6, and 7, represent various conditions in which this normal 

 process has not occurred. Figures 7 and 1 1 are later stages of this type 

 of abortion in which degeneration of the nucleus and cytoplasm is grad- 

 ually taking place. Figure 15 apparently represents a further stage of 

 this series, in which the nucleus, although visible in outline, is, with the 

 cytoplasm, entirely functionless. Plate E, figure 11, shows the com- 

 pletely degenerate microspore of this type found among normal mature 

 pollen. The dark-staining mass is yellow and oily before killing. 



