PLANT CELL GROWTH AND NUTRITION 459 



TABLE IV 



Relative Abundance of Water and Protein Molecules in Cultured Carrot Cells 



Micrograms No. of H^O No. of Protein HgO Molecules per 



% H2O per Cell Molecules Molecules* Protein Molecules 



90% 0.10 to 0.15 Order of 3 x lO^^ Order of 10^ Order of lO" 



** The number of protein molecules is estimated on the basis of a molecular weight of 

 64,000. If their average molecular weight were greater— say 640,000— the number of molecules 

 would be proportionally fewer. 



characteristics of the organism upon so much water with so little dry 

 matter! 



From the discovery of plasmolysis (by Carl Nageli, 1855) and its 

 recognition as a special case of osmosis (by Wilhelm Hofmeister, 

 1867), attention became focused upon the relatively brief e.xchanges 

 between cells and their environment v^ith respect to water. To achieve 

 success in the interpretation of the water relations of vacuolated cells— 

 the simple osmotic view— virtually required that attention should be 

 confined to cells in being, in contrast to those originating by division 

 and throughout their growth, and it also required that the exchanges 

 of solutes between cells and their environment be ignored, at least for 

 the duration of the observations. It is therefore not surprising that the 

 period that produced the works of Pf offer (1877), de Vries (1871), and 

 Graham and M. Traube (1867) saw the first emphasis upon the per- 

 meability of cellular membranes as the main control over the passage 

 of solutes into and out of cells. But even so, Overton, the great ex- 

 ponent of permeability theory, sounded a word of warning and stressed 

 that such molecules as sugars, amino acids, and inorganic ions (pro- 

 verbially slow to move across the boundary surfaces of cells through 

 their "passive" permeability) were nevertheless physiologically neces- 

 sary, so that their intake into the cells was determined by a sort of 

 secretory activity, for which the term "adenoid activity" was coined. 

 With the passing of time, Collander, one of Overton's distinguished 

 followers and the outstanding modern student of cellular permeability 

 in plants, has found it difficult to attach much physiological significance 

 to the passive permeability phenomena (Collander, 1959). Indeed, 

 Collander now regards all of the most operative movements of physi- 

 ologically important solutes as being determined by active transport 

 mechanisms. In such mechanisms the cellular organization must inter- 

 vene to augment, or to override, the movements that would occur by 

 diffusion alone. 



