134 DIFFUSION AND OSMOTIC PRESSURE 



In the second series of cultures, besides the normal and 

 modified Knop's solutions, two non-electroytes, cane sugar 

 and lactose, were used, and it was found that here also 

 the concentration is the controlling factor in the response of 

 the plant. It must be noted, however, that to prevent the 

 formation of filaments a somewhat higher concentration is 

 required of these sugars than of the inorganic salt. This 

 is perhaps due to a more ready absorption of the sugars 

 and consequent rise in internal concentration. Whatever 

 may be the cause of this phenomenon, it seems to be in accord 

 with that noted by van Rysselberghe, 1 namely, that cells of 

 Tradescantia, etc., develop a greater turgidity in salt solu- 

 tions than in those of sugar. 



It is thus shown conclusively that the changes in the 

 growth of this plant which result from changes in the con- 

 centration of the medium are entirely dependent upon its 

 osmotic pressure. This means that they are dependent upon 

 the amount of water contained within the cells, for the strong 

 solutions extract water, while the weak ones allow it to be 

 absorbed. 



In a weak solution vegetative growth is very much more 

 rapid than in a strong one. This may be due to the fact 

 that in a strong solution the water content of the protoplasm 

 is reduced in amount below the limit for optimum lability. 

 When the plant grows fastest and best it is in the filamen- 

 tous form. In the weak solution, where activity seems to be 

 at a maximum, the ions of the electrolytes, which are essen- 

 tial for metabolism, are not plentiful. This may suggest how 

 the cylindrical form of cell with its increased surface 2 may 



1 VAN RYSSELBEBGHE, '" Reaction osmotique des cellules v6g6tales a la concentra- 

 tion du milieu," Mem. cour., pub. par 1'Acad. roy. de Belg., Vol.LVIII (1898), pp. 1-101. 



2 In a cylinder, the lateral surface is greater than that of a sphere of the same 

 volume, as long as the ratio of the length to the diameter equals or exceeds 2.727. 

 In typical filament cells of this alga, the ratio of the diameters is 3, and it is often 4 

 and even greater. It is seldom less than 2.8. Thus, it is shown that the filament cell 

 offers more surface to the surrounding medium through its lateral walls alone than 

 does the palmella cell of equal volume. 



