A STUDY 01? RECIPROCAL CROSSES. 89 



appearances precisely alike, germinative tests were made to note 

 any differences in rapidity and per cent of germination. For 

 this study sugar solutions of I, 2, 3, 5, and 10 per cent were 

 taken as culture media. It seems remarkable that plants which 

 are so different in gross structure should show absolutely no 

 difference in structure of their pollen, or in per cent and rapidity 

 of germination. In all cultures, however, the protoplasm 

 streamed out in much the same manner, and measurements were 

 made of the length of pollen tubes, but even here no difference 

 could be detected. 



A study of pollen of yellow plum was furthered by making 

 close-pollinations, without emasculating, and covering the flow- 

 ers to prevent access of foreign pollen. In nearly every case the 

 pollen did its. work, for the fruit set and came to maturity. 

 Treated in a similar way flowers of red currant gave similar 

 results. 



(2.) Trouble with pistils. 



In view of the foregoing study and its results, the writer was 

 convinced that the trouble lay, not with the pollen, but probably 

 with the pistils, or in combining yellow plum pollen and red cur- 

 rant pistUs. By this time the process of emasculation was begun, 

 and here great difficulty was experienced. It seemed almost 

 impossible to emasculate the red currant without destroying the 

 upper portion of the pistil. This trouble led to a comparative 

 study of the pistils of red currant and yellow plum. This com- 

 parison has revealed the major reason for inability to cross the 

 plants. 



The pistil of the red currant is considerably smaller than the 

 yellow plum, as can be seen with only a superficial examina- 

 tion ; but a more careful study reveals the fact of minor differ- 

 ences which are of considerable importance. The most striking 

 difference, aside from that of size, is to be found at the junction 

 of style and ovary. At this point, in the case of red currant, there 

 is a constriction in the pistil, which is much narrower and is 

 placed in a slight depression in the ovary. A critical examina- 

 tion, however, shows no joint, although there is a slight dissim- 

 ilarity in size and structure of cells. The style of the red cur- 

 rant is, then, largest half way between the ovary and stigma, 

 that is, it tapers toward the stigma and also toward the ovary. 



