THE MATERIAL INCOME OF PLANTS 309 



The condition of cell walls in a massive tissue may be comprehended clearly by 

 inspecting a mass of bubbles such as may be formed by blowing air through a tube 

 into a soap solution. 1 The outer bubbles will have a convex surface, but plane 

 films divide the air bubbles in the interior. Pricking a superficial bubble gives 

 opportunity for the plane walls of those adjacent to it to bulge, because the in- 

 ternal pressure is now unbalanced. 



A cell thus overfilled with water, with the elastic wall stretched, or 

 under strain and ready to stretch, is said to be turgid, and the condition 

 is designated as turgidity. Manifestly, turgidity depends upon two fac- 

 tors : the presence of a solute or solutes in sufficient amounts, and an 

 adequate supply of water. 



Turgor and osmotic pressure. — The pressure developed within the 

 cells, when an adequate amount of water is at hand, may equal the os- 

 motic pressure of the solutes to which the cytoplasm is impermeable. 

 Obviously, the osmotic pressure exists, whether or not it exhibits itself; 

 it exhibits itself by stretching the elastic container only when sufficient 

 water can enter; this particular exhibition of it is known as turgor? or 

 turgor pressure. Thus within the cell there exists both osmotic pressure 

 and turgor pressure; the latter is a sort of hydrostatic pressure depend- 

 ent upon the former for its existence and probably upon the resistance 

 of the protoplast and the cell wall to filtration for its amount. It is 

 seldom likely, therefore, to equal the osmotic pressure. 



Thus, in the cells of the sugar beet, the cane sugar alone has an osmotic pressure 

 of 10 or 11 atmospheres; and there are certainly many other solutes which would 

 add greatly to this. But the turgor pressure can only reach a point at which water 

 will filter through the cytoplasm and cell wall, and this is probably less than half the 

 osmotic pressure of the sugar alone. 3 



That the osmotic pressure is always ready to produce turgor is shown 

 by the fact that flaccid cells placed in pure water quickly become 

 turgid. 



Plasmolysis. — If a turgid cell is placed in a solution more concentrated 

 than that within it, water emigrates from the cell, which then becomes 

 more or less flaccid. By measuring turgid cells, or making careful 



1 This may be made of a plain glycerin soap. More durable bubbles may be made 

 from this solution : Shaved white Castile soap 10 gm. by weight; warm water 400 cc; 

 dissolve. To 15 parts by volume, acid glycerin 11 parts. This will be improved by al- 

 lowing it to stand for a week, cooling over night to ,s° C. and filtering cold until limpid. 



2 The term is not always thus restricted ; it is often used ;is synonymous with turgidity. 



3 Further studies of this subject are much needed, especially as the usual mode of test- 

 ing osmotic pressure by plasmolysis has been shown to be faulty. 



