A STUDY OF RECIPROCAL CROSSES. 85 



A. Incomplete Development of Pollen Tubes. 



(1.) Impotent pollen or poisonous stigmatic fluid. 



The study of this feature of the problem had its origin mainly 

 with the investigations of Darwin 1 and no systematic study of 

 self-sterility was taken up till after the publication of Origin of 

 Species in 1859. While horticulturists and other investigators 

 were studying the causes of sterility for many years, it was not 

 till quite recently that the histological study of sterility was 

 begun. Even though the general causes of sterility have been 

 pretty well worked out, yet the details of the problem, — the non- 

 development or incomplete development of the pollen tubes, — is, 

 so far as the writer's knowledge is concerned, still unexplained. 



It is well at this point to refer to the investigations of the 

 pioneers on this subject. Darwin, for example, found that in the 

 case of legitimate and illegitimate union of the elements in 

 Linum perenne, if pollen of either form be placed upon its own 

 stigmas, the pollen grains would germinate and the tubes enter 

 the tissue of the pistil, but to what extent is not known. Darwin 

 observed here that the impotency of the pollen must be due either 

 to the tubes not reaching the ovules, or to their improper 

 action upon reaching them. 



Again, in the case of Linum grandiflora where other unions, 

 legitimate and illegitimate were made, it was ascertained by 

 examining the pistils that the pollen of illegitimate unions ger- 

 minated very rarely, and in those where germination did take 

 place the tubes were short and penetrated the tissue of the pis- 

 til only a short distance. 



In other cases the pollen seems to be absolutely impotent, and 

 in such cases the trouble seems to lie in the inability of the stig- 

 matic secretion to properly excite the pollen in such a way as to 

 form tubes. This has been found to be the case with some of the 

 Orchidaceae where self pollination results in the death of the 

 flower. Fritz Miiller 2 says that pollen masses and the stigma of 

 the same plant in various species of Orchidaceae have actually 

 a deadly effect upon one another, as in the case of Oncidiuni 

 oscrolrilum. This was shown by the surface of the stigma in con- 

 tact with the pollen, and by the pollen itself becoming dark and 



1. Darwin. Different Forms of Flowers on Plants of the Same Species. Chap. 

 1,2,3. 



2. Fritz Miiller, Bot. Ztg., 1868, p. 114. 



