PHYSIOLOGY 465 



concerned in this are the surface cells of young rootlets, and espe- 

 cially the root-hairs (Fig. 451). In these plants, where the exposed 

 cells are cuticularized to prevent loss of water into the air, the 

 entrance of oxygen and carbon-dioxide is provided for by the 

 development of stomata, which communicate with the intercellular 

 spaces. The gases, entering the latter, are brought into contact 

 with the thin- walled cells, by which they are absorbed. Water 

 and gases may be excreted by the active cells into these intercellular 

 spaces, and from them be discharged through the stomata. The 

 impermeability of the cell-wall is usually the result of a change of 

 the cellulose into cork or some similar substance. This change 

 may completely destroy its power of imbibition. Such changes are 

 associated with the exposure of the cells to a dry atmosphere, and 

 are universally found in the epidermal cells and outer cortex of 

 terrestrial plants, and on wounded parts exposed to the air. 



The living protoplasmic membranes are not only less permeable 

 than the cellulose wall, but their permeability is probably much 

 more variable. This is functionally important for retaining within 

 the cell certain dissolved substances which would otherwise pass out 

 and be lost. It also allows for the accumulation of large amounts 

 of soluble matter from very dilute solutions, such as exist normally 

 in the soil or water when the plant is growing. The accumulation 

 of substances within the cell-sap is easily shown by exposing absorb- 

 ent cells {e.g. root-hairs of various water-plants, cells of Vallisneria, 

 etc.) to a dilute solution of methylene blue, which soon accumulates 

 so that the cell-sap is deeply colored. Sometimes a precipitate is 

 formed, or it is otherwise rendered incapable of passing out of 

 the cell. 



Oils, fats, etc., are capable of passing through membranes, prob- 

 ably in the form of a fine emulsion, i.e. in excessively fine drops 

 mixed with water. 



Translocation of Substances 



While in unicellular -or filamentous water-plants the substances 

 absorbed may be at once used by the cells into which they pass, 

 in the higher plants this is usually not the case, and the substances 

 must be carried long distances before they are used. While dios- 

 mosis is the principal means by which solutions pass through living 

 tissues, it must be remembered that there are frequently protoplas- 

 mic connections between the cells, which must play some part in 

 the transfer of substances from one cell to another, as well as being 

 the means of transmitting stimuli of different kinds. Occasionally, 

 as in sieve-tubes, the communications between the cells are suffi- 

 ciently evident, and permit of the passage of small solid bodies, such 

 as starch-grains, as well as solutions. 



2H 



