442 



ELEMENTARY BIOLOGY 



This diagram led to the suspicion that what had been known 

 under a single name was in reality a mixture of two different 



plants. In order to try out this 

 idea, de Vries selected seeds from 

 heads having thirteen ray-flowers, 

 and planted them separately. The 

 rays on the flowers of the new 

 plants derived from these seeds 

 were counted, and the distribution 

 of the variations found to cluster 

 more closely around thirteen rays, 

 as shown by the dotted line in 

 the figure. This would show that 

 the wild daisies represented prob- 

 ably two distinct races, or strains, 

 that looked enough alike to be 

 considered as a single species. 



Another illustration is furnished 

 by some beans studied by a Danish 

 scientist, Dr. Wilhelm Johannsen. 

 From the appearance of the beans, 

 and from the appearance of the 

 curve showing the distribution of 

 their variations, no person would 

 suspect that they were not all of 

 one kind. But when Dr. Johannsen 

 planted single seeds of different 

 sizes, and kept the plants carefully 

 guarded against possible cross- 

 pollenation, he obtained groups of 

 seeds that clustered around the 

 respective parent seeds in size. 

 This process was continued for several generations, until 

 the experimenter succeeded in separating nineteen pure lines 

 of plants that remained distinct in following generations. 



8 '9 10 11 12 13 14 15 16 17 1 18' 19 20 2122 



Fig. 2-V7- Two-peaked curve of 



237- 



variation 



The height of the line above the base 

 corresponds to the number of daisy 

 heads {CIny santJieynztrn segetiirn) hav- 

 ing the number of ray-florets indi- 

 cated at the bottom. The solid line 

 shows that there were many heads 

 with 13 florets, and very many with 

 21, and that the intermediate num- 

 bers were represented by fewer indi- 

 viduals, as were also the numbers 

 above 21 and the numbers below 13. 

 The broken line shows the distribu- 

 tion of the following year's crop, 

 grown from seeds out of 13-ray heads. 

 Note the greatly increased proportion 

 of 13-ray heads 



