iv.] NATURE'S EXPERIMENTS 35 



bowl of water. In a few days all the leaves 

 adapted to air perished, but the stems at once 

 proceeded to develop new dissected leaves, but 

 adapted to live submerged (fig. 6). 



Nature frequently performs analogous experi- 

 ments, for small ponds may dry up for a season, 

 and if the water crowfoot grows in it, it 

 flourishes equally well on the mud in air, if 

 not better than in water. If the crowfoot grows 

 very crowded, it often happens that branches 

 get thrust up into the air. When this occurs 

 the anatomical structure changes at the water 

 level, all above it being in adaptation to air 

 as occurs often with R. trichophyllus (fig. 7). 



The values of these experiments reside first in 

 the rapid or immediate response made by the plants 

 to totally new conditions, which strengthens the 

 induction that the submerged type of leaf, like 

 the aerial one, is induced to be formed by the 

 external conditions. But experimental verifica- 

 tion is not wanting. Mr MacCallum, of the 

 United States, starting with the a priori assump- 

 tion that the plant is unable to complete the 

 full development of the leaf, in consequence of 

 the super - saturation of the living substance, 

 protoplasm ; and in order to test this " work- 

 ing hypothesis" he made the water denser by 

 dissolving nutritive substances in it. These he 

 assumed would, by " osmotic " action, cause the 

 extra water to be withdrawn from the protoplasm ; 



