DEPARTMENT OF BOTANICAL RESEARCH. 71 



(4) If pieces of the same size and shape as above are cut from freshly 

 made gelatine and hung on a thread with the long axis in a vertical 

 position, where they are exposed to equal evaporation on all sides, the 

 same distribution of decrease in size and subsequent increase in water 

 takes place, as was found under paragraphs 2 and 3. 



(5) If gelatine is poured into a large dish or on a glass slab and 

 allowed to lose water by evaporation before pieces are cut for tests, 

 the decrease in thickness far exceeds the decrease in the other direc- 

 tions, and the subsequent swelling when pieces are placed in water 

 follows the same proportion. For example, 15 per cent gelatine when 

 treated in this manner showed swelling to the following amounts: 

 thickness 181 per cent, breadth 15 per cent, length 6 per cent. 



Other more or less obvious precautions must be heeded in order to 

 obtain comparable results, and by no means least of these is the neces- 

 sity for obtaining all the gelatine from the same source and of making 

 frequent tests for variations in acidity. It has been known for some 

 time that the greater the surface exposed the more rapid the intake of 

 water, but it was necessary to find how much this fact needed to be 

 taken into account in obtaining comparable results. It was found that 

 pieces of the same area might vary in thickness from 0.01 to 0.36 cm. 

 without showing an appreciable difference in percentage rate of in- 

 crease in thickness, total volume, or gain by weight. On the other 

 hand, pieces from 0.01 to 0.23 cm. in thickness, whose areas varied 

 from 1.62 sq. cm. to 1.82 sq. cm., showed difference in rate of increase 

 for at least 48 hours, but after this time the total increase was the same, 

 whether measured by height, volume, or weight. 



The abo^'e conclusions have been given at some length not only 

 because they must be taken into consideration in all experimentation 

 \Yith the swelling of gelatine, but also because it seems that they 

 place an entering wedge into the problem of the mechanism of the 

 absorption of w^ater bj^ gels. Furthermore, they give a new view of 

 the complexity of the factors which may operate to control rate 

 and direction of growth as well as absorption and transpiration in the 

 highly complex colloids of the plant. 



The Physical Basis of Parasitism, by D. T. MacDougal. 



The earlier experimental studies of the author led him to con- 

 clude that a necessary condition of parasitism was a higher osmotic 

 concentration of the species which could become parasitic. Harris 

 and Lawrence have carried out an extensive investigation of the prob- 

 lem on Loranthaceous parasites in the Jamaican 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 character- 

 ized by a higher osmotic concentration of its fluids. They also show 

 that on theoretical grounds higher osmotic pressure of the tissue fluids 



