PLANT BREEDING AND EVOLUTION 175 



DARWIN'S EXPERIMENTS IN INBREEDING AND CROSSING 



Darwin laid the foundation for all modern experimentation 

 in cross-pollination and sross-fertilization of plants in his book 

 on " Cross- and Self-Fertilization in the Vegetable Kingdom." A 

 detailed account is there given of painstaking and elaborate 

 experiments which he performed on a great variety of plants. 

 Darwin's methods were essentially similar to those pursued 

 to-day in the breeding of plants by cross-pollination and hybridi- 

 zation. Some flowers were either self -pollinated (with their 

 own pollen) or close-pollinated (with pollen from another flower 

 on the same plant). Others were cross-pollinated by placing 

 the pollen from one flower on the stigma of a flower on a 

 different plant of the same species. The plants were then pro- 

 tected from cross-pollination by insects or wind by means of fine 

 cloth stretched over frames so as to cover the plants and protect 

 them from insect visitors. In modern practice the anthers are 

 removed from the flowers on the pollinated plants, and they 

 are then inclosed in parchment or paper bags to exclude pollen 

 borne by insects or wind (Fig. 93). 



After the plants had formed their seeds, Darwin collected 

 the seeds from the self-fertilized and cross-fertilized species 

 and estimated their relative number and weight in all cases. 

 The seeds from the self -fertilized and the cross-fertilized plants 

 were then sown, and the relative vigor of the offspring de- 

 rived from the two kinds of seeds was noted and accurately 

 measured. Darwin proved by these laborious methods that 

 fifty-seven species, or kinds, of plants produced more vigorous 

 offspring as a result of cross-fertilization than resulted from 

 close-fertilization of plants belonging to the same species. 



The nature of Darwin's methods and results is so important 

 that the following tables and diagram have been selected from 

 his book, with an extract from his summary of results. 



The first diagram and the accompanying conclusions relate 

 to an experiment with the common morning-glory (Ipomoea 

 purpurea), in which ten generations of plants were cross- 

 pollinated. The second table and summary give similar results 



