128 



BOTANICAL GAZETTE 



[august 





of the cell. The pentosans play their most important part when 

 they are in the form of minute particles in the colloidal mesh of 

 living matter, in which they are still subject to positive but slow 

 changes in metabolism. According to MacDougal, Richards, 

 and Spoehr, 2 the conversion of other sugars into the pentosans, 

 greatly increasing the hydration capacity of the protoplasm, is the 

 basis of the origination of the xerophytic and succulent types of 

 vegetation. 



These facts warrant a procedure in the study of swelling and 

 growth in which various pentosans and pentosan-protein mixtures 

 were subjected to the action of solutions, applied in a concentra- 

 tion and with variations and alternations parallel to occurrences in 

 the cell. 



The method of measuring the water capacity of colloidal 

 material includes the following features : 



i. Suitable solutions or suspensions are poured at tempera- 

 tures of 30 to 40 C. into shallow molds to form a jelly. 



2. The plates thus cast are dried in a small chamber with a 

 high relative humidity, to a thickness of o. 1 to o. 25 mm. 



3. Trios of sections 3X5 mm. in area are placed in dishes of a 

 capacity of 30 cc, a triangular piece of thin glass is placed over 

 the sections, and the vertical swinging arm of an auxograph is 

 seated in the center of the glass. Any change in thickness of the 

 sections moves the levers and moves a pen on a recording sheet 

 carried by the drum. 



4. Solutions, the effect of which on the hydration capacity of 

 the sections are to be tested, are poured into the dishes. In 

 nearly all of the experiments in the special range of biological 

 relations it is advisable to renew or replace the solutions at certain 

 intervals. 



5. Temperature relations are of the greatest importance, and 

 the record is taken by mercurial thermometers, the bulbs of which 

 are in dishes of liquid similar to those of the experiment. 



2 MacDougal, D. T., and Spoehr, H. A., The origination of xerophytism. 

 Plant World 2 1 : 245 . 1919. 



MacDougal, D. T., Richards, H. M., and Spoehr, H. A., The basis of succulence 

 in plants. Bot. Gaz. 67:405. 1919. 







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