642 Journal of Agricultural Research voi. xii, no. 10 



Oenothera lata the early abortion of the male generative organs and 

 partial abortion of the female is in some way associated with the presence 

 of an extra chromosome, while in Oenothera semilata an extra chromo- 

 some is present, but only a portion of the pollen grains abort. Morgan 

 has shown that slight mutations are continually occurring in Drosophila 

 spp., which inhibit the development of the 2X individual, while Bridges 

 (5) has shown that certain chromosome combinations can not bring 

 about normal development in the zygote. Is it too much to expect 

 that like conditions may affect the i X generation also ? 



Dorsey {12) pointed out a different type of pollen degeneration from 

 that which expresses itself in the production of empty grains. He 

 showed that in functionally pistillate grapevines pollen developnemt 

 proceeds normally through the microspore division and the formation 

 of the normal content of cytoplasm found in mature fertile grains. 

 During the period of development following the microspore division, 

 one or both of the nuclei of a portion of the pollen grains aborted, leaving 

 the grain normal as far as cytoplasm was concerned. Associated \^^th 

 the complete sterility of the pollen of the functionally pistillate varieties 

 were the reflexed type of stamens, an entire lack of sutures and germ 

 pores in the mature pollen, and dieciousness. It is probable that the 

 lack of germ pores is the direct cause of stei-ility in the numerous grains 

 which otherwise appear entirely normal. Tischler, Rosenberg, and others 

 have shown that pollen abortion in hybrids may follow either normal or 

 irregular reduction divisions, when the parents have both an equal and 

 unequal number of chromosomes. Shull (35, 36) has recorded some 

 consistent irregularities in sex ratios in Lychnis dioica which will later 

 be shov/n possibly to have been due to slight mutations causing pollen 

 abortion. Goodspeed {21, 22) gives further evidence on the sterility of 

 hybrids oi Nicotiana spp,, when A^ sylvestris is used as one parent andshovv^s 

 that not only the pollen is sterile but that the F^ plants are incapable of 

 forming any very appreciable amounts of seed. Rimpau {31) and 

 Jesenko (26) have shown that in hybrids between wheat and r5'e there is 

 complete male sterility, while some of the egg cells are able to produce 

 viable seed if pollinated with either wheat or rye pollen. 



The cytological investigation of pollen development in the strawberry, 

 reported in this paper, has two main objects in view: (i) the determina- 

 tion of the mechanism and nature of pollen abortion and \vith these 

 facts at hand, (2) the determination to which of the many categories of 

 sterility the very prevalent pollen abortion in the strawberry varieties 

 belongs. 



Material and methods. — The variety used as the basis of this study 

 is Minnesota 3. As was previously stated, it furnishes desirable ma- 

 terial for this type of study, as about 50 per cent of its pollen aborts, 

 while the remainder develops normally. The egg cells in this variety 

 and in the cultivated varieties in general do not show a corresponding 



