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. 

 The- height of the line above the base From the appearance of the beans, 



corresponds to the number of daisy &nd from ^ app earance of the 

 heads (Chrysanthemum segetum) hav- 

 ing the number of ray-florets indi- CUrVC showing the distribution of 

 cated at the bottom. The solid line th j variat i ons no pe rson Would 

 shows that there were many heads 



with '13 florets, and very many with SUSpCCt that they Were not all of 

 21, and that the intermediate num- ^ ^ But when Dr. Johannsen 

 bers were represented by fewer indi- 

 viduals, as were also the numbers planted single seeds of different 



^?S. b SSS: sizes ' and ^ the P lants carefu "y 



tion of the following year's crop, guarded against possible cross- 



f?rh f ^eX^llSro S n pollenation, he obtained groups of 



of i 3 -ray heads 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. 



UJJJJJJJnUJJJJ 



FIG. 237. Two-peaked curve of 



variation 



