230 STUDIES IN GENEKAL PHYSIOLOGY 



which the substances contained naturally (calcium carbonate, 

 magnesium sulphate, etc.) were present to the point of saturation. 

 The course of the experiments always showed that the salts con- 

 tained in the water, and the presence of which had been demon- 

 strated chemically had no particular effect. 



In spite of this, Semper concludes 



that some substance must be present in the water, probably in 

 exceedingly small amounts, which, through its solution in the water, 

 and its osmotic behavior toward the skin of the animal, is absorbed 

 by the latter in definite, though perhaps small, amounts. 



It seems to me that an important factor has not been 

 recognized in these experiments. An excess of food (Elodea 

 canadensis) was probably present in all the dishes, but it 

 was not observed whether all the animals ate the same 

 amounts a factor upon which the results depend altogether. 

 When I was raising butterflies I noticed how readily young 

 caterpillars lose their appetites (especially immediately after 

 hatching). In such cases the growth of the caterpillars of 

 the same brood varies according to the amount of food they 

 take up. It is quite possible that, if this point is taken into 

 consideration, the experiments of Semper find a simple 

 explanation. 



I undertook my own experiments on growth to determine, 

 first of all, whether the mechanics of growth is the same in 

 animals as in plants. In plants it is believed that water 

 enters the cells osmotically, that the cell- walls are stretched 

 in consequence, and that through further changes this 

 stretching of the cell-walls becomes permanent, and remains 

 even when the intra-cellular pressure has again fallen off. 

 If the water furnished a plant is decidedly reduced, longi- 

 tudinal growth is diminished and finally stopped entirely. 

 I have described an experiment in the preceding volume on 

 experimental morphology which shows that those parts of 

 the animal which have lost their turgidity are just as little 

 , able to grow as wilted plants. 



