ABSOEPTION. 565 



of evaporation, as when plants are grown under a bell-glass. Rauweii- 

 hoff also states that absorption exceeds transpiration in amount in propor- 

 tion as the pressure is greater, and where the latter is slight evaporation 

 is in excess. 



Root-action. It has been shown that absorption of fluids takes 

 place near the extremities of the finest rootlets above the inert root- 

 cap, and by means of the root-hairs. But the water in the soil is 

 often nearly pure water, or, at least, contains little admixture of 

 mineral matter, though more or less impregnated with gases. 

 Again, many of the ingredients of the soil, such as silica for in- 

 stance, are insoluble in pure water. How then do these substances 

 gain access, as we know they do, to the interior of the plant? They 

 must be derived from without and in a liquid form. The explana- 

 tion now given is, that the minute rootlets and root-hairs insinuate 

 themselves between the particles of soil, absorb the water there 

 situated with its minute proportion of dissolved mineral matter. 

 Further, these root-hairs come into close contact with the minute 

 particles of soil, and, by virtue of some processes of excretion not 

 yet thoroughly examined, they excrete a substance, gaseous or liquid, 

 which effects the solution of the mineral substance in the particle 

 of soil. An exhalation of carbonic dioxide from the roots, con- 

 joined with the water in the soil, would effect the solution of lime, 

 for instance. In this way the furrows and impressions on the sur- 

 face of marble made by roots is explained. The opinion that root 

 excretions exist, at one time denied, may thus be correct ; but as 

 they appear to be only excreted when required, and are used up in 

 the process of solution, their presence in the soil is not manifested 

 any more than that of the gastric juice in an empty stomach. 



Absorption by Leaves. The leaves and other green parts of the 

 higher plants do not appear, as a rule, to absorb liquids all the 

 time the roots are in action, but if root-absorption be insufficient 

 then absorption by the leaves takes place. The good effect of 

 syringing plants in hothouses seems rather to depend on the check 

 given to undue evaporation than to absorption, the structure of the 

 epidermis being generally unfavourable for that process. Whether 

 leaves absorb even watery vapour to any great extent is question- 

 able ; but it is certain that they absorb gases, including ammonia, 

 though, under ordinary circumstances, only in very small quantities 

 (Mayer), and that a very large proportion of the carbon which is 

 consumed by green plants is taken into the system, in the form of 

 carbonic dioxide gas, by the leaves and green shoots. 



The entrance of gases into the cells is attributable, through their 

 solubility in water, to endosmotic action ; while the laws of diffusion 

 of gases provide for their entrance into the intercellular passages, 

 which brings them into contact with the deeper-seated cells. 





