50 THE BIOLOGICAL PROBLEM OF TO-DAY 



order, by artificially shading the upper surface, and 

 strongly illuminating the lower surface. 



Among the most interesting heteroraorphoses are 

 the galls, produced upon young plants when certain 

 insects lay eggs on them, or when plant-lice irritate 

 their tissues. From these abnormal stimuli there 

 result active masses of cells which grow into organs 

 of definite form and of complex structure. The 

 galls, moreover, differ widely, in correspondence 

 with the specific stimulus which was their initial 

 cause, and with the specific substance, the stimula- 

 tion of which resulted in the formation of a gall. 

 By the action of different insects upon the same 

 plant different galls are produced, and the galls of 

 different plants may be distinguished systemati- 

 cally. 



Blumenbach has already brought forward the 

 existence of galls as an argument against preforma- 

 tion, holding them to be structures produced epi- 

 genetically, and, therefore, unrepresented by rudi- 

 ments in the germ. I, also, consider them witnesses 

 against Weismann's gerrnplasm. They teach us 

 that the cells of the plant- body may serve purposes 

 quite different from those arranged for in the course 

 of development ; that cells modify their form in 

 correspondence with novel conditions, and that 

 they are forced into forming special structures, not 

 by special determinants in the germ, but by external 

 stimulants. 



Galls exhibit yet another instructive kind of 

 heteromorphosis. 



Even the tissue of a leaf, turned into a gall by 



