Animals and Plants. 179 



at first sight, be supposed to depend upon a structural difference 

 between the two " sub-kingdoms " to be mentioned presently — 

 the general presence of a rigid cell-wall in the plant and its 

 fairly constant absence in the animal. Given an outer cell- 

 wall, the taking in of nutritious matter must necessarily be 

 limited to the process of osmosis. If two fluids (or gases) of 

 different specific gravity be separated from each other by an 

 animal membrane, it will be found, after a time, that the two 

 fluids have, both of them, passed through the membrane. The 

 amount which has passed from one side to the other depends 

 upon their different specific gravities. This is purely physico- 

 chemical osmosis. In living animals and plants the same 

 phenomenon is met with, but the living protoplasm is sup- 

 posed to alter the purely physical nature of the diffusion. It 

 has been shown that the presence of the cell-wall is not the 

 only reason for the fact that a true plant does not take in solid 

 particles of food, for in many motile unicellular plants (such as 

 Hmmatococcus), where the protoplasm is at times in the plant's 

 life perfectedly naked, no protrusion of pseudopodia and 

 absorption of particles has been noticed. 



On the other hand, it must be remembered that the animal 

 body is also fed by similar process of osmosis. The food taken 

 into our stomachs is chiefly thus absorbed. But many of the 

 foodstuffs used by us are non-diffiisible substances. This is the 

 case, for example, with albumen, such as white of egg. What 

 happens is that the stomach and other regions of the gut secrete 

 a digestive juice such as gastric juice, succus entericus, etc., 

 which converts these indiffusible substances into diffiisible ones. 

 They can then be absorbed by osmosis. This, however, again, 

 is not distinctive of the animal as opposed to the plant. The 

 insectivorous plants have been already referred to. In them a 

 juice is thrown out which actually produces the same effects 

 upon the proteids of a fly's body, and converts its insoluble 

 proteids into soluble peptones, which are then absorbed. 



We may conclude, however, by emphasizing the fact that, 

 while plants generally live upon inorganic and organic salts, 

 animals never do ; that plants generally absorb their carbon 

 from the atmosphere by the help of their chlorophyll, while 



