No. 578] 



SELF-STERILITY 



S3 



of the tubes in the selfed and the crossed styles is wholly- 

 one of rate of growth. The tubes in the selfed pistils de- 

 velop steadily at a rate of about 3 millimeters per twenty- 

 four hours. There is even a slight acceleration of this 

 rate as the tubes progress. If the flowers were of an 

 everlasting nature one could hardly doubt but that the 

 tubes would ultimately reach the ovules, though this would 

 not necessarily mean that fertilization must occur. Since 

 the maximum life of the flower is about 11 days, however, 

 the tubes never traverse over one half of the distance to 

 the ovary. On the other hand, the tubes in the crossed 

 pistils, though starting to grow at the same rate as the 

 others, pass down the style faster and faster, until they 

 reach the ovary in four days or less. 



From these facts it seems reasonable to conclude that 

 the secretions in the style offer a stimulus to pollen tubes 

 from other plants rather than an impediment to the de- 

 velopment of tubes from the same plant. 



The whole question, therefore, becomes a mathematical 

 one, that of satisfying conditions whereby no stimulus is 

 offered to pollen tubes from the same plant, but a positive 

 stimulus is offered to tubes from nearly every other plant. 



Morgan has given an answer to this question in a gen- 

 eral way. If I understand his position correctly, my own 

 conclusions are not very different from his, but are some- 

 what more definite. Morgan (1913) states that the re- 

 sults of Adkins and himself on Ciona intestinalis can best 

 be understood by the following hypothesis : 



The failure to self-fertilize, which is the main problem, would seem 

 to be due to the similarity in the hereditary factors carried by the eggs 

 and sperm; but in the sperm, at least, reduction division has taken 

 place prior to fertilization, and therefore unless each animal was 

 homozygous (which from the nature of the case cannot be assumed 

 possible) the failure to fertilize can not be due to homozygosity. But 

 both sperm and eggs have developed under the influence of the total or 

 duplex number of hereditary factors; hence they are alike, i, c, their pro- 

 toplasmic substance has been under the same influences. In this sense, 

 the case is like that of stock that has long been inbred, and has come 

 to have nearly the same hereditary complex. If this similarity decreases 



