DESICCATION IN PHILODINA ROSEOLA. 381 



not entirely regain the normal condition. It would be inter- 

 esting to know if the plant would have recovered if given more 

 time for the process. In Philodina the process of recovery begins 

 upon the addition of water and is complete in a short time, 

 never longer than a few hours. 



For an analysis of the meaning of these changes I should say 

 that the cactus is unfavorable for rapid results. The great 

 length of time necessary for changes to take place and the slow- 

 ness with which normal conditions are resumed makes experi- 

 mental work with a large number of specimens difficult. This 

 probably accounts in part for the fact that McDougal and Brown 

 make no attempt to suggest the significance of the cytological 

 changes which they describe. 



Metabolic Water and Its Relation to Desiccation. 



For a complete discussion of the subject of metabolic water 

 and its functions in an organism I would refer the reader to the 

 excellent paper of Babcock ('12). For the present purpose it 

 will be sufficient to outline the main points of the subject and 

 relate them to the material under discussion. 



Desiccation is the operation of drying or removing water 

 from a substance. When the last trace of water is removed the 

 desiccation is said to be absolute. Absolute desiccation of 

 chemical crystals is brought about by heating the substance to 

 dryness or when the material involved is readily decomposed 

 by heat the operation is accomplished by the use of a desiccator. 

 A condition of absolute desiccation can be maintained so long 

 as the heat is applied in the one case or so long as the substance 

 is kept within the drying chamber in the other. When the 

 desiccating influence is removed moisture is absorbed. 



Inorganic crystals can, as a rule, be deprived of their moisture 

 only by a temperature of 100 degrees centigrade or higher. 

 Not only are the crystals as such made dry but the water of 

 crystallization, by virtue of which the crystals exist, is driven 

 off and the substance assumes an amorphous condition. 



Living organisms contain water combined with them in a 

 fashion which for present purposes may be compared to water 

 of crystallization. In order to remove all water by means of 

 heat it is necessary to raise the temperature of the organic 



