146 VEGETABLE PHYSIOLOGY 



gypsum, a polished plate of marble is inserted into the 

 flower-pot, after a certain time of growth of the plant con- 

 tained in it, the plate will exhibit a tracing of the course 

 of the roots which have come into contact with it, but, 

 instead of being in relief as in the former case, it will be 

 etched to a certain depth. The solvent influence can thus 

 be seen to come from the root itself, and not the water in 

 the soil. It will, in fact, be the acid sap which makes its 

 way out of the root -hairs. 



Certain constituents of the soil can be absorbed which 

 are made available in neither of the ways mentioned. 

 Soils contain many constituents which cannot pass through 

 the plasmatic membranes of the protoplasm, but which, in 

 the presence of water, react with one another, producing 

 new compounds which are capable of such osmotic entry 

 and which are consequently absorbed. 



The solutions taken in are excessively dilute. We 

 cannot make a plant take up a greater quantity of any 

 salt by bringing its roots into contact with a strong solu- 

 tion of it. There is a certain relation necessary between 

 the substance and the water, which has been the subject of 

 considerable investigation. Every salt is absorbed by a 

 particular plant in a certain strength of solution, or in other 

 words with each molecule of salt there is a certain invari- 

 able quantity of water taken in. The quantity is not the 

 same, however, for each salt. 



The salts which different plants absorb, in like manner 

 vary in amount. If two species are growing in the same 

 soil, side by side, under exactly the same conditions, 

 the amounts of the several salts present in the soil which 

 are absorbed by the plants of the different species will not 

 be the same. In each case the quantity will vary accord- 

 ing to the use the plant can make of it. This is well 

 illustrated by the amounts of silica which can be taken up 

 by grasses and by leguminous plants respectively. In an 

 ordinary pasture there are always found several kinds of 

 grasses, together with clover and other allied plants. An 



