STOUT AXD SUSA, CHROMOSOME IRREGULARITIES 13 



fold or crease along one side in the long axis. In the sections (see text 

 figure) certain grains are cut in their short axis and hence appear out of 

 proportion to the grains that lie with the long axis in view. A consider- 

 able number of the grains are undersized, shrivelled and of irregular 

 shapes. Nearly all of the cells have the exine with its reticulations de- 

 veloped to some degree, and relatively few of these spores are entirely 

 lacking in protoplasmic contents. 



The Nuclei and the Chromosome Number in Microspores 



In many of the microspores, at the time anthers are about to dehisce, 

 there is a single nucleus, which is in the resting condition and which 

 possibly does not develop further. But division figures for the formation 

 of the vegetative and the generative cells are to be found. In some of 

 these 6 chromosomes are present (PI. Ill, Fig. 36) ; in others more than 

 6 are to be seen ; but thus far the largest number noted has been 18 (PI. 

 Ill, Fig. 37). In all these divisions thus far observed only rod-shaped 

 chromosomes have been found. The two-celled stage of the microspore 

 following the division of the primary nucleus is rather frequently seen, 

 with the smaller generative cell lying to one side of and entirely outside 

 of the vegetative cell. In many such microspores further development 

 evidently does not take place. 



The Viability of the Pollen 



Many tests for the germination of pollen have been made over a period 

 of ten years and involving many plants of the Europa clon obtained 

 from various widely separated countries. There is excellent germination 

 of about 3% of the pollen grains on a medium of 1% agar with 10% or 

 15% cane sugar. In no test has more than 5% of all the microspores 

 germinated and only well-formed grains of good size have germinated. 

 The viable pollen grains make fine long tubes and, as pollen is abundant 

 in the anthers, it would seem that sufficient viable pollen is produced to 

 yield seed freely when there is a compatible relation in fertilization. 



The Abortion of Macrospores 



The abortion of the macrospores has not been studied in detail by 

 cytological methods, but the behavior of plants of the Europa clon 

 in respect to seed-setting indicates that abortion is quite the same in 

 degree for macrospores as for microspores. To all self-pollinations there 

 is complete self-incompatibility; the ovaries make no start toward de- 



