PARTHENOGENESIS AND PURE LINES 207 



ubers, cuttings, etc. East has studied the effect of selec- 

 ion of tubers of certain races of the common potato. A 

 •ace was first grown from a single tuber. By boring holes 

 nto the tubers enough material could be obtained for a 

 chemical test of the amount of nitrogen in them. The 

 •est of each tuber could, if desired, be cut into pieces of 

 standard size and planted. Ten tubers, high in nitrogen, 

 md ten, low in nitrogen, were selected. The tubers of the 

 lext generation showed that there was no relation found 

 )etween the amount of nitrogen in the original tuber and 

 n those that came from it. A repetition of the experi- 



FiG. 95. — A wingless aphid to the left and a winged to the right, both belonging to 

 the same species. (After Webster and Phillips.) 



nent in another generation gave only meagre results 

 )wing to drought. As far as the facts went, this genera- 

 ion, too, showed no effect of selection. 



Most of the protozoa propagate by dividing into equal 

 )r nearly equal parts — i.e., by a process of cell-division, 

 lennings has studied the effect of selection in a culture 

 )f Paramecium, all members of which had descended from 

 I single individual. No change was induced. Later, how- 

 ever, working on another protozoon, Difflugia corona, 

 Jennings found that selection brought about changes in 

 :he direction of selection. In this case, the method of 

 iivision may possibly include irregular distribution of the 

 chromatin material, and the recent work of Hegner indi- 

 cates that such an interpretation is not improbable. Pos- 



