178 PART in: PHYSIOLOGY. [ 42 



pot, and a plant be grown in the soil above it, the roots will come 

 into contact with the slab and will apply themselves to its surface. 

 On subsequently examining the slab of marble, it will be found to 

 have become corroded where the roots had been in contact with it. 

 The corrosion is due to the solution, by the acid sap of the roots, 

 of particles of the marble. 



Absorption of Liquids. The main idea connected with this 

 function is the taking up of water and other substances into the 

 plant from without ; but it must not be overlooked that, in a 

 multicellular plant, the cells absorb from each other. 



In any case, the function of absorption depends upon the 

 physical process of diffusion through membrane of substances in 

 solution, or osmosis. Tor instance, supposing two adjacent cells, 

 one of which has its cell-sap charged with sugar, whereas that of 

 the other has none ; the sugar will diffuse through the intervening 

 cell-wall until the sap in both cells holds the same proportion in 

 solution. This being the mode of absorption, it is clear that the 

 food-materials can only be absorbed in the fluid form, either as 

 liquids or gases. 



So far the function of absorption would appear to be a simply 

 physical process. It must, however, be borne in mind that the 

 cell-wall is lined by living protoplasm which modifies the purely 

 physical diffusion through the cell-wall, both as regards the nature 

 and the relative quantity of the substances which pass into or 

 out of the cell ; so that the physical laws of osmosis, as determined 

 by experiments with dead membrane, are not directly applicable 

 to the osmotic phenomena of a living cell. 



Absorption of Gases. The absorption of gases depends, like the 

 absorption of water and substances in solution, upon diffusion. 

 Supposing an absorbent cell, the sap of which holds, to begin with, 

 no gases in solution, to be brought into relation with a mixture of 

 gases, these gases will be dissolved at the surface in proportion to 

 their solubility and to the amount of each gas present in the mix- 

 ture ; that is, the amount absorbed of each gas depends, in the first 

 instance, upon its solubility and its partial pressure. The relative 

 amount of each gas absorbed over a period of time will further 

 depend upon the extent to which it undergoes chemical alteration 

 after absorption. 



Land-plants absorb gases, in the manner described above, at all 

 points of their surface ; by their shoots from the air, by their roots 

 from the gaseous mixture in the interstices of the soil ; the stomata 



