IMPORTANCE OF WATER 123 



these combinations too may determine the establishment of Don- 

 nan's equilibria between the cell and the surrounding medium and 

 may promote the accumulation in the plant of many anions and 

 cations in much higher concentrations than are found in the sur- 

 rounding medium ; for instance, in the soil solution. At the pres- 

 ent time, attempts are being made to reconstruct the whole theory 

 as to the entrance of substances from the soil into roots on the 

 basis of the conditions discovered by Donnan. 



39. Changes in the Colloidal State of Protoplasm and Its 

 Influence on Osmotic Properties. Coagulation and Death of 

 Protoplasm. — The property of not being permeable to substances 

 dissolved in the cell sap is preserved by protoplasm only as long as 

 it is alive. As soon as protoplasm dies or is killed, it loses its 

 impermeability, and the substances dissolved in the cell sap dif- 

 fuse out. The cooking of food is based to a considerable extent 

 on the property of killed cells releasing the substances contained 

 in them. This refers especially to fruits and vegetables which in a 

 raw state are less digestible, as a considerable number of their cells, 

 remaining unbroken during mastication, pass through the ali- 

 mentary canal without yielding up the enclosed nutritive sub- 

 stances. 



The fixation of the protoplasm for subsequent staining of the 

 enclosed nucleus, chondriosomes, etc., for cytological study is 

 based on the loss of permeability by the cells. The living proto- 

 plasm is impermeable to almost all stains except a few used for 

 staining in the living condition: neutral red, methylene blue. To 

 differentiate and to stain the component parts of a cell, it is neces- 

 sary to kill it by a poison such as alcohol, acetic and osmic acids, 

 etc., which will not disturb the minute structure of all these forma- 

 tions. 



The loss of impermeability by the killed cell results from changes 

 in the state of the colloids of the protoplasm. From the condition 

 of a sol they are transformed to that of a gel, the degree of their 

 dispersion decreases, and their micellae form irregular aggregates 

 between which canals, readily accessible to water and solutions, are 

 opened. This phenomenon is called "coagulation." It may be 

 easily illustrated by heating the white of an egg, which is some- 

 what diluted with water. 



The coagulation of protoplasm may be brought about by dif- 

 ferent causes, such as high temperature, poisons, salts of heavy 



