CHAPTER V. 

 THE ABSORBING SYSTEM. 



/. GENERAL CONSIDERATIONS. 



All the materials that are taken into the plant-body from the surround- 

 ing medium must enter in the liquid condition or in a state of solution. 

 It is true that plants can render solid bodies available for absorption 

 with the aid of enzymes and acid substances which they secrete for 

 that purpose ; such solvent actions come into prominence during the 

 germination of albuminous seeds, and also play an important part in 

 the so-called digestive processes of carnivorous plants. But in these 

 cases one is after all dealing with changes which go on outside the 

 plant-body, and which merely prepare the solid substances for absorp- 

 tion. In fact, if the phenomena of fertilisation, and the ingestion of 

 solid matter which takes place in the Myxomycetes, be left out of 

 consideration, it may be stated quite generally that foreign material 

 cannot penetrate into the interior of a living cell except by 

 diosmosis. 



Among the substances which are absorbed by all plants, water 

 occupies a unique position. A comparatively small amount of this 

 substance is needed for nutritive purposes, in order, that is, to provide 

 the hydrogen and oxygen which enter into the composition of most 

 organic compounds. A somewhat larger quantity is required to serve 

 as the so-called water of imbibition which saturates protoplasm, cell- walls, 

 starch grains, and so forth, and to provide the bulk of the cell-sap. 

 By far the greatest proportion, however, of the water absorbed, by 

 land-plants at any rate, is needed to make good the loss due to 

 transpiration. 



All the other materials which are normally absorbed by a plant 

 represent, from the nutritive point of view, either indispensable food- 

 constituents, or else so much useless matter, which is taken in merely 

 because the organism, though possessed of a certain amount of selective 



