460 AMERICAN JOURNAL OF BOTANY [Vol. io, 



fundamental principle operating in their origin and development. In 

 these higher plants there is the development of a nearly naked egg (free of a 

 decided membrane) imbedded in the tissue of the ovule, and a highly 

 specialized male gametophyte— the pollen tube — with its various growth 

 relations in the style. While fertilization in these cases involves a series of 

 physical and chemical relations, it is fundamentally an egg-and-sperm 

 reaction. 



It is to be noted that studies of the physiology of pollen germination 

 have failed to reveal a specificity that is comparable or related to the com- 

 patibility or incompatibility found in homomorphic species. It has, how- 

 ever, long been known that in cases of self-incompatibility the pollen tubes 

 often make feeble growth in the style, and more recently it has been shown 

 that their rate of growth is not accelerated as the eggs are approached. 

 This condition apparently persists when incompatible pollen is mixed with 

 compatible pollen and the tubes of both grow side by side in the style. 

 This indicates that the reactions are decidedly discriminative and suggests 

 that they involve reactions of the pollen tubes to secretions of the eggs. 

 There is, however, evidence that in many grades of self-incompatibility the 

 injurious effects may be exhibited after what is apparently a successful 

 fertilization. 



One aspect of the development of self-incompatibility in the hermaphro- 

 dite is clear. Every such individual is itself the result of a compatible 

 fertilization in which two cells fused, and the two elements of the resulting 

 diploid organization show themselves compatible throughout the somatic 

 life of the individual; but the two kinds of sex elements produced by it are 

 incompatible, and this is of course irrespective of their germ-plasm complex. 

 The incompatibility arises along with sex differentiation, which in seed 

 plants begins with the development of pistils and stamens and is independent 

 of any readjustments of the germ plasm in the reduction divisions. 



The biogenetic nature of the development of self-incompatibilities is 

 further indicated by the wide variations which exist in their expression in 

 individuals. Between the extreme or alternative conditions there are many 

 intergradations, and the extreme conditions are reversible in a progeny. 

 This is the general rule of behavior in such pedigreed cultures as have been 

 critically tested in this particular. Cross-incompatibilities exhibit quite 

 the same ranges of expression, and here reciprocals may give directly 

 opposite results. 



But there is also conclusive evidence in certain species of cyclic changes 

 in the self-compatibility of an individual. These are best seen in plants 

 which have a somewhat extended and continuous period of bloom. In 

 some species there is self-compatibility at the end of the period of bloom ; 

 in others the climax of self-compatibility is at the mid-period of the bloom, 

 and for certain perennials there is some evidence of changes from year to 

 year in relation to the age of the plant. It is to be noted that a mid-period 



