THE PHYSICAL BASIS OF PARASITISM 



243 



TABLE 1 ■ 

 Swelling of mixtures of protein and agar 



WATER 



HUNDREDTH 

 MOLECULAR 



FIFTIETH 

 MOLECULAR 



TENTH 

 MOLECULAR 



Potassium nitrate 



per cent 



3266 



per cent 



1800 



per cent 



1733 



per cent 



1333 



Calcixim nitrate 



1333 



1200 



800 



possible which may cause water absorption in great volume 

 independently of osmosis and in fact in opposition to it.^ 



The penetration of a host by the haustorimn of a parasite is 

 not to be regarded as conditioned by the simple osmotic balance 

 between two tracts of cells of equivalent physical condition. 

 The invaded tracts of the host are usually composed of expanded 

 vacuolated cells in which osmosis resulting from the solutions 

 in the vacuoles is the dominant hydrostatic agent, although the 

 colloids suspended in these vacuoles, and the denser colloids 

 of the cytoplasm have their own imbibitional capacities. 



The younger cells of the haustorium which push into such 

 masses are probably not yet vacuolated. Absorption by them 

 is almost wholly by imbibition and this would be carried on 

 against any probable osmotic action of a vacuolated cell. Thus 

 a thin plate of "plant colloid " mixture absorbed water from a 

 solution of potassiimi nitrate which had an osmotic coefficient 

 of 60 atmospheres, and swelled about 400% in volume in fifteen 

 hours. 



A second feature, the force of expansion of the invading proto- 

 plasts, would be no less important. The pressure set up, like 

 that of a swelling seed, would be great enough to cause me- 



3 See MacDougal, D. T. Imbibitional Swelling of Plants and Colloidal Mix- 

 tures. Science, N. S., 44: 502-505, October 6, 1916. 



Also, MacDougal, D. T. and Spoehr, H. A. The Behavior of Certain Gels 

 Useful in the Interpretation of the Action of Plants. Science, N. S. 45: 484^S8, 

 1916. 



