A. B. Stout 99 



even in the same flower, may be as incompatible in function as though 

 they were produced by plants of unrelated genera. 



There has been much speculation as to the nature and operation of 

 the physiological processes operating in such incompatibilities as are 

 seen in physiological self- and cross-sterility. In many cases of self- 

 incompatibility it has been reported that there is a limited or restricted 

 growth of pollen-tubes. These facts have led to views that the de- 

 termining factors in compatibility and incompatibility are limited to 

 the relations between pollen-tubes and pistils alone. Jost (1907) con- 

 siders that the poor growth of pollen-tubes in such cases is due to 

 the action of individual stuff which inhibits growth of pollen-tubes 

 having the same stuff. Compton (1912, 1913) believes that self- 

 sterility is due to the absence of a stimulating stuff, the presence of 

 which gives fertility. East (1915 a) attributes self-sterility to absence of 

 food stufis which are not secreted because the pollen-tubes involved do 

 not possess any hereditary element not possessed by the diploid cells 

 of the pistil. Moore (1917) considers that the limited growth in length 

 of pollen-tubes observed in self-sterility in Tradescantia is really due 

 to the presence of too much food. 



Some of these views appear to regard the determining factors as 

 conditions of the pistil alone ; others consider that the conditions arise 

 through a reciprocal reaction between tubes and pistil. All of them 

 fail to recognize that a critical period in the growth of the pollen-tube 

 may result from secretions of the egg, and that the diflferent qualities 

 of the pistil may be due to the diffusion of hormones from the gameto- 

 phytes. As I have earlier pointed out, there is some evidence that 

 some cases of embryo abortion may be due to incompatibility, expressing 

 itself after fertilization and during the development of the embryo. 

 This may be true in some- cases in chicory. Further studies are in 

 progress on this point. 



Cross-sterility (within a species) without self-sterility might be 

 explained as are isoprecipitation phenomena on the basis of an intra- 

 specific specificity of individuals, or groups of individuals, as such. 

 Self-compatibility, however, shows that an equally- marked differential 

 specificity may develop in sex organs and gametes produced by a single 

 individual : such specificity is not characteristic of the sporophytic 

 individual as a whole, but of the pollen-tubes, pistils, embryo sacs and 

 eggs as such. 



