Mar. II, 1918 Sterility in the Strawberry 651 



Pollen maturity. — After the microspore division there is again a 

 period of growth of the pollen grain and a very marked increase in the 

 amount of cytoplasm. Eventually the pollen grain is completely filled 

 mth cytoplasm of a distinctly alveolar structure. During this time the 

 cytoplasm nearly disappears from about the microspore nucleus, leaving 

 the microspore cell wall loosely surrounding the nucleus, the chromatin 

 of which is graduallv taking on the condition typical of resting nuclei 

 (PI. D, 2). 



When pollen formation is complete, changes take place in the anthers 

 preparatory to dehiscence. These consist primarily in breaking down 

 the wall between each pair of loculi, thus throwing all of the grains of 

 one-half of the anther together. There is also a general drying-out 

 process which results in the disappearance of the liquid material which 

 previously surrounded the developing grains, and of a considerable 

 portion of moisture from the pollen grains, thus causing them to collapse 

 along three meridial sutures which fold in, thus giving in cross section 

 somewhat of a clover-leaf shape, while the general shape of the grain is 

 long-oval. The three germ pores are located at the middle points of the 

 sutures. On placing the dry pollen in water it soon swells to form a 

 sphere. 



The position of the generative cell and vegetative nucleus within the 

 collapsed grains could not be determined, for as soon as the killing fluid 

 penetrated the anthers the grains immediately swelled. In the swelled 

 grains the nuclei lie in various positions with regard to one another, but 

 usually in close proximity. The vegetative nucleus is generally spherical 

 while the generative cell contents are' fusiform, owing to the folding of 

 the dried pollen grains and are not closely surrounded by the cell wall. 

 The chromatin of both nuclei now shows the typical resting condition 

 (PI. D, 2). 



Up to and including the liberation of the microspores from the tetrad, 

 the cells have shown marked regularity in division, the stages of short 

 duration proceeding regularly from one end of a loculus to the other, 

 while during the stages of longer duration the cells of a loculus all show 

 the same degree of development. Up to the time of liberation of the 

 microspores, the development which has taken place has depended 

 entirely on materials furnished by the sporophyte, the one group of 

 chromosomes merely being the tools by which the materials of the 

 pollen mother cell were divided into four parts, the microspores. So 

 far as can be seen up to this point, no growth process resulting directly 

 in an increase of cell material can be directly attributed to the one aggre- 

 gation of chromosomes. At the time of liberation from the tetrad 

 the spores are strikingly alike in size and all other visible characters. 

 There has been, up to this time, no differentiation in rate of development 

 of individual microspores; and the spores, as liberated, are all normal. 



