THE PHYSICAL BASIS OF PARASITISM 241 



Harris and Lawrence have carried out an extensive investi- 

 gation of the problem on Loranthaceous parasites in the Jamai- 

 can rain forests, and in a paper now in press have shown that 

 in case of plants growing under these conditions the parasite is 

 generally but not invariably characterized by a higher osmotic 

 concentration of its fluids. They also show that on theoretical 

 grounds- higher osmotic pressure of the tissue fluids is not a 

 necessary prerequisite of successful parasitism in the case of a 

 species living under natural conditions. - 



That the absorbing organs of a plant might withdraw liquids 

 from tissues of another plant, the sap of which had a higher con- 

 centration, is also to be concluded from the results of recent 

 work on imbibitional phenomena at the Desert Laboratory. 



Extended series of measurements established the fact that a 

 mixture consisting of 90% or more of agar and 10% or less of 

 protein, albumen, gelatine, tyi'osin or cystin, takes up water 

 in a manner remarkably parallel to that of pieces of tissue 

 of living plants. This similarity is regarded as more than a 

 coincidence. The plant protoplast consists largely of carbohy- 

 drates of the pentosan group, with which are mixed varying 

 proportions of nitrogenous material w^hich may be in the form 

 of protein, amino-acids, etc. Such a mixture would have identi- 

 cal relations to w^ater either as sw^elling plates in the labora- 

 tory, or as water-absorbing sheets or strands of colloid in the 

 cell. 



A number of agencies or conditions are found to affect the 

 total amount of water which may be taken up by this "plant- 

 colloid " mixture. Thus, for example, nearly all of such mix- 

 tures absorb slightly more water in acidified solutions than in 

 alkaline, and many times as much from neutral as from either 

 acid or alkaline solutions. Some salts in the solution increase 

 imbibition and some lessen it. 



These generalizations rest upon measurements made by the 

 following method : Small sections of dried plates of a mixture of 

 "plant-colloids " were placed in trios in glass dishes into which 



2 Report Dept. Bot. Res. Carnegie Inst, of Washington for 1916, pp. 79 and 80. 



