230 STUDIES IN GENERAL 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 (EHlodea 
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. 
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