406 MEMOIRS OF THE NEW YORK BOTANICAL GARDEN 



of sex differentiation operating in the individual rather than to 

 racial or line characteristics or stuffs for the development of 

 conditions, local or quite general for the plant, which determine 

 its relative fertility. 



This is fully realized by East, who states with reference to his 

 own experiments with tobacco already discussed in the intro- 

 duction, ''When these experiments were begun, I expected to 

 find that the facts would accord with a simple dihybrid ]\Iendelian 

 formula similar to that which Correns later proposed as an inter- 

 pretation of his results. Yet only by considerable stretching and 

 a vivid imagination will Correns' data fit such a hypothesis, and 

 my own data do not fit at all " ('1 5a, p. 82). East's interpretation, 

 although recognizing that there can be no hereditary factor 

 directly representing fertility as such,' assumes that the direct 

 conditioning substances are in the germ plasm. He considers that 

 the physiological relations are mainly those of pollen-tube growth, 

 involving interaction between the somatic cells of the pistils and 

 the pollen tubes, and the favorable reaction is assumed to occur 

 only when the haploid nuclei of the pollen tube possess at least 

 some one factor which is not present in the nuclei of the diploid 

 cells of the pistil. This relation, purely somatic at least on one 

 side, he calls "gametic incompatibility," a term which hardly 

 expresses the processes assumed to be involved. 



It is especially to be noted that Darwin, Correns, and East 

 seek the causes of sterility in a lack of differentiation of the 

 gametes; it is too great a similarity that prohibits fertility. With 

 Correns' no plant can be homozygous for any one line stuff: 

 with East no plant should be self-fertile since its pollen grains can 

 possess no element of the germ jjlasm which is not found in the 

 somatic cells of the pistil of the jilant on which the pollen grain is 

 formed. The narrow applicaiiility of East's theory is most evi- 

 dent. It ignores the facts which Burck has emphasized so strongly 

 and the abundant evidence, discussed later in this paper, that it 

 is similarity of germ plasm elements that favors fertility. Ac- 

 cording to East's view self-fertility should not occur in any plant, 

 there should be no close approach to homozygosity such as he 

 has previously pointed out is the rule in tobacco, and there should 

 never be the development of self-fertile plants from self-sterile, 

 unless the hereditary units of the germ plasm are subject to 



