436 MEMOIRS OF THE NEW YORK BOTANICAL GARDEN 



precipitation, for example, an excess of antigen frequently inhibits 

 precipitation, a phenomenon quite analogous to the inhibition 

 zones in colloidal flocculations in which the relative quantity is 

 more important than the chemical nature of the substances. 

 The whole theory of precipitation and agglutination phenomena 

 rests on an assumed differentiation in the chemical or physical 

 properties of the products of individuals or of different organs of 

 the individual. The reaction occurs, it is assumed, because of 

 dissimilarity. One could conceive of such conditions arising from 

 the differentiation of sexual organs borne by the same plant and 

 of fluctuations that would allow for such variations in self- and 

 cross-fertility and sterility as do appear in chicory. Thus far, 

 however, there are no well-marked or general data as to the 

 behavior of isoagglutinins or isoprecipitins which are entirely 

 parallel to the cases of incompatibility giving self-sterility. That 

 the phenomena do indicate that sexual compatibility is propor- 

 tional to similarity of the organisms or gametes rather than to 

 dissimilarity or differentiation is, however, strongly suggested. 



The phenomena of pollen-tube growth. — Extensive experiments 

 have been conducted to determine the physiology of pollen 

 tubes both in reference to direct growth in pistils and their extracts 

 and to their behavior in different methods of artificial culture. 

 The knowledge thus gained bears on the problems of sterility, 

 especially from the standpoint of an understanding of (i) the 

 requirements for germination, (2) the extent to which pollen-tube 

 growth is determined by physical or local conditions or by its own 

 initiative, in contrast to (3) the extent to which its growth is 

 influenced by direct secretions of the ovule with its Ggg. The 

 results with different species show such wide differences that it is 

 difficult to establish any general rule of behavior in respect to any 

 one of these questions. 



The older opinion that specific chemical substances or condi- 

 tions are necessary for germination of pollen is hardly tenable in 

 view of the very general germination of pollen on pistils of widely 

 different species (especially reported by Strasburger 1886, antl by 

 Tokugawa '14), and of the very fre(iuent ()l)servation that germi- 

 nation occurs in pure water (JNIolisch '93; Lidforss '96), and that 

 a proper control of the water supply is often all that is necessary 

 to secure germination e\ t-n in cases in which llu- difficulty of 



