ABSORPTION OF WATER. 65 



absorption will take place quite independently of its use 

 or its hurtfulness to the plant. We have also learned that 

 the amount of any substance absorbed depends upon its 

 relation to the metabolism of the plant. We can imagine 

 that a substance might pass readily into the absorbent cells 

 and be distributed from cell to cell throughout the plant, 

 without entering at all into the metabolic processes ; sodium, 

 as we see from Wolff's analyses, affords a case in point. The 

 result of the absorption of such a substance would be that 

 the cell-sap of the cells would soon reach such a point of 

 saturation as regards this substance that diffusion-equilibrium 

 would be set up, and then no more of it would be absorbed. 

 The case of a salt which does enter into the metabolic pro- 

 cesses is very different : it undergoes decomposition, and some 

 constituent of it is frequently thrown down in the insoluble 

 form. The effect of this is that a constant withdrawal of the 

 salt from the sphere of osmotic action is taking place, and 

 thus a demand for it is set up which is met by the absorption 

 of fresh quantities of it by the roots from the soil. It is thus 

 quite possible to give a satisfactory explanation of the facts 

 observed without making any unwarrantable assumptions. 



We will conclude this lecture with an examination of 

 the existing evidence as to the absorption of water and of 

 substances in solution by organs other than roots. In those 

 Thallophytes which possess no root-hairs, absorption is effected 

 by the cell or cells which constitute the thallus. Submerged 

 Cormophytes doubtless absorb, to some extent at any rate, by 

 their general surface, and in Salvinia the peculiarly modified 

 aquatic leaves are the only absorbent organs which the plant 

 possesses. In these cases the leaves are especially adapted 

 for the purpose, notably in this respect, that the walls of their 

 epidermal cells are not cuticularised. This is true also of the 

 glands of the leaves of the so-called carnivorous plants which 

 absorb organic substances in solution. It has been thought, 

 though, by many, that the ordinary foliage-leaves of land- 

 plants are capable of absorbing water, and that to such an 

 extent as to constitute an important source of supply. This 

 is a point which has excited considerable interest, and it 



V. 5 



