654 Journal of Agricultural Research voi. xii, no. io 



Although there has been a considerable amount of degeneration up to 

 the time of the completion of growth of the microspore wall, probably 

 more takes place between the time of the formation of the large vacuole 

 (Pi. C, 8) and the completion of the microspore division than at any 

 other period. 



Plate D, figure 12, shows a full-grown microspore in which degeneration 

 is taking place, both in the cytoplasm and in the nucleus. This was found 

 in an anther containing full-grown i -nucleate microspores, and probably 

 is a case in which degeneration has begun during the period of enlarge- 

 ment. 



Plate D, figure 13, and Plate E, figure 2, represent early stages in de- 

 generation of I -nucleate microspores which are at the stage at which 

 division should take place as they were found in anthers containing both 

 I- and 2-nucleate grains, as well as division stages. The microspore rep- 

 resented in Plate E, figure 7, was found among microspores of the stage 

 of development shown in Plate C, figure 15, while that of Plate E, figure 

 8, was found among mature pollen grains. Both are evidently later 

 stages of the condition represented in Plate D, figures 12 and 13, and Plate 

 E, figure 2. 



During the period of division a few microspores contain a very scant 

 amount of cytoplasm. Such spores are seen in Plate D, figures 14 and 16, 

 and Plate E, figures i and 4. Eventually these completely degenerate. 

 Those of the type shown in Plate D, figure 16, and Plate E, figure i , prob- 

 ably are the forerunners of the completely degenerate microspore shown 

 in Plate E, figure 10, which is a common type in mature anthers. 



Although degeneration takes place to a greater or less degree at all 

 stages, from microspore liberation to microspore nucleus division, no 

 instances of degeneration occurring during the process of division have 

 been noted. Directly following division, however, evidences of degener- 

 ation again become apparent, although much less numerous than in the 

 period just prior to division. Figure 3 of Plate E represents an early 

 stage of degeneration directly following microspore division. This is 

 probably an early stage of the more advanced degeneration stage shown 

 in Plate E, figure I. This young pollen grain was found among grains in 

 which the cytoplasm was increasing rapidly in amount. It will be noticed 

 that the generative cell is aborting, while the remainder of the grain 

 is normal. Apparentiy grains of this type continue to increase normally 

 with regard to the vegetative portion, while degeneration of the genera- 

 tive cell occurs. Figure 1 3 of Plate E represents such a grain found in a 

 mature anther. Figure 12 represents another type of degeneration 

 which takes place subsequent to division of the microspore nucleus. The 

 generative cell has completely degenerated. The vegetative nucleus, 

 although still present in outline, is f unctionless, while the cytoplasm bears 

 no resemblance to the normal. It still retains the property, however, of 



