How Plants Draw in Various Materials 193 



and from places where they occur to places where they are being 

 absorbed. ^Moreover, each kind of gas diffuses by itself, no mat- 

 ter what others may be present, so that a gas in process of ab- 

 sorption by a plant can move inward in a steady stream through 

 another which is not being absorbed, and even against the op- 

 posite stream of one in process of release. Thus, in photosynthe- 

 sis, for example, a constant current of carbon dioxide diffuses 

 into the leaf, through nitrogen which remains without move- 

 ment, against a current of oxygen which is diffusing outward. It 

 is a condition hard to imagine, it is true, but the facts declare it 

 is so. The gases thus mipelled along the passages by diffusion 

 finally reach the living cells, and, being soluble in water, are 

 dissolved by the moist surfaces, and then diffuse through walls 

 and protoplasm to the places of use. And here I may add a sug- 

 gestion, for the benefit of the reader versed in physics, that this 

 movement of different gases in contrary directions along the same 

 passages is explained much better by the old-fashioned idea that 

 diffusion and gas pressure are caused by a mutual repulsion 

 between the same kind of molecules than by the modern kinetic 

 theory, which makes those phenomena the result of vibratory 

 movements of the molecules; and moreover the very same con- 

 ception explains perfectly how osmotic pressures and gas pres- 

 sures can be identical in kind as well as in quantity. 



A very special case of absorption occurs in those plants which 

 absorb organic food substances already made. Such plants, of 

 which parasites are a good example, have the power of excreting 

 from their absorbing parts those special enzymes, or ferments, 

 which render soluble the organic materials they touch. The 

 dissolved substance then enters the plant by diffusion from the 

 place of high concentration outside to the places of use and low 

 concentration inside, — the intermicellar spaces, of course, being 

 adjusted for the admission of these large molecules. The ab- 

 sorption by pollen-tubes of tissues through which they pass; of 

 hmnus by the fungi which live thereupon; and of the materials 



