stout: pollinations in cichorium intybus 345 



of pollen tubes in self- and cross-fertilization. This Is an attempt 

 to discriminate degrees or grades of differentiation through the 

 immediate requirements and Interactions between the sex organs 

 of a single plant and of different plants. Such considerations, of 

 course, involve the extensive studies that have been made upon 

 the physiology of pollen tubes In the attempt to determine their 

 irritability and nutritive requirements and to what extent their 

 development is determined by direct stimulation of the egg. 



Jost ('07) was the first to summarize such evidence in formu- 

 lating a theory of self-sterility. He points out from the results 

 that he and others have obtained In experimental studies of pollen 

 germination and pollen-tube growth, that the different species 

 exhibit a wide variation with more or less marked specific physio- 

 logical differentiation, which we may note is quite In harmony 

 with the more recent studies. The stimulating influence of sugars, 

 or proteids to pollen- tube growth, and the fact that a regulation 

 giving a very limited supply of water Is all that is necessary for the 

 germination and early growth of pollen tubes Is an evidence that 

 the growth of pollen tubes in the pistil, as in artificial cultures, is 

 quite Independent to a certain degree of the direct influence of 



the eggs. 



Within the species, however, self-sterility may arise from quite 

 Individual conditions Irrespective of any specific physiological 

 differentiation. In such cases Jost points out: {a) the pollen may 

 not germinate, (b) the growth of tubes for a plant may be poor In Its 

 own conducting tissues, (c) the tubes may not respond to chemo- 

 tropic stimuli in tissues of its own pistil, (d) the two sex cells may 

 not be able to unite, (e) the product of their union may make very 

 poor development, or that several of these conditions may com- 

 bine to give sterility. All of these classes except the last I have 

 Included under the class "physiological self-incompatibility;" any 

 failure In development after fertilization I have classed as "em- 

 bryo abortion" for reasons already stated. 



These limitations of fertilization operating so decidedly In the 

 individual are made the basis of the application by Jost of the old 

 doctrine of "Individual stuffs" which Is, perhaps, the first formal 

 hypothesis which has been advanced to account for the phenomena 

 of physiological self-incompatibility. The theory Is in brief that: 

 (a) Different plants even of the same species and strain are 

 characterized by qualitatively different chemical substances; 



