in Hybridisation 337 



The experimental plants must necessarily 



1. Possess constant differentiating characters. 



2. The hybrids of such plants must, during the flower- 

 ing period, be protected from the influence of all foreign 

 pollen, or be easily capable of such protection. 



The hybrids and their offspring should suffer no marked 

 disturbance in their fertility in the successive generations. 



Accidental impregnation by foreign pollen, if it occurred 

 during the experiments and were not recognized, would 

 lead to entirely erroneous conclusions. Reduced fertility 

 or entire sterility of certain forms, such as occurs in the 

 offspring of many hybrids, would render the experiments 

 very difficult or entirely frustrate them. In order to dis- 

 cover the relations in which the hybrid forms stand towards 

 each other and also towards their progenitors it appears to 

 be necessary that all members of the series developed in 

 each successive generation should be, without exception^ 

 subjected to observation. 



At the very outset special attention was devoted to the 

 Leguminosae on account of their peculiar floral structure. 

 Experiments which were made with several members of 

 this family led to the result that the genus Pisum was 

 found to possess the necessary qualifications. 



Some thoroughly distinct forms of this genus possess 

 characters which are constant, and easily and certainly 

 recognizable, and when their hybrids are mutually crossed 

 they yield perfectly fertile progeny. Furthermore, a dis- 

 turbance through foreign pollen cannot easily occur, since 

 the fertilising organs are closely packed inside the keel 

 and the anther bursts within the bud, so that the stigma 

 becomes covered with pollen even before the flower opens. 

 This circumstance is of especial importance. As additional 

 advantages worth mentioning, there may be cited the easy 

 culture of these plants in the open ground and in pots, 

 and also their relatively short period of growth. Artificial 

 fertilisation is certainly a somewhat elaborate process, but 

 nearly always succeeds. For this purpose the bud is 

 opened before it is perfectly developed, the keel is removed, 

 and each stamen carefully extracted by means of forceps, 

 after which the stigma can at once be dusted over with 

 the foreign pollen. 



B. H. 22 



