118 DIFFUSION AND OSMOTIC PRESSURE 



II. ABSORPTION OF DISSOLVED SOLIDS AND LIQUIDS 

 It is probable that the protoplasmic membranes of plant 

 cells are, normally at least, slightly permeable to all sub- 

 stances which the organism needs to absorb. We have 

 direct evidence of the permeability of protoplasm to many 

 of these substances ; this evidence was presented in chap, i 

 of this Part. How it comes about that a cell may retain 

 turgor and still be permeable to solutes was also discussed 

 there. The substances which are diffusing into a plant cell 

 at any moment cannot be the ones which are producing the 

 turgor pressure. "While certain organic molecules are main- 

 taining the turgor, for instance, many inorganic ions may be 

 diffusing into the cell, because the partial diffusion tension 

 due to them is lower within than it is without. It is also 

 possible that permeability changes from time to time, so that 

 a substance which cannot penetrate the protoplast at one time 

 may do so at another. Turgidity is maintained, and the 

 protoplasmic layer kept stretched and in contact with the 

 imbibed cellulose walls, by the osmotic pressure of certain 

 substances which are probably formed within the cell and to 

 which the protoplast is but slightly permeable. 1 



Thus it is clear that the absorption of solid and liquid 

 solutes from the surrounding solution is, like that of gases, 

 merely a phenomenon of diffusion, the particles moving 

 toward that part of the solution where lowest diffusion ten- 

 sion of that substance obtains. Water plants possibly absorb 

 solutes through all parts of their submerged surfaces. Land 

 plants can absorb them only where they are in contact with the 

 moist substratum, mainly through the roots and root hairs. 



The so-called power of selection of absorbing organs 

 deserves some attention here. It is observed that some 

 plants absorb much more of certain substances than others, 



1Q. H. VON MAYENBUKG, " LOsungsconcentration und Turgorregulation bei den 

 Schimmelpilzeu," Jahrb.f. wiss. Bot., Vol. XXXVI (1901), pp. 381-420. 



