Preface. v 



characteristic of swelling in colloids, and the term hydration is used in 

 the present work to include the entire range of action. 



The method of study employed has been one in which biocoUoids 

 have been compounded from pentosans and proteins in proportions 

 simulating those of the plant, and the total range of swelling of thin 

 plates of this material has been measured by the auxograph which has 

 been developed for this purpose. Series of measurements of such 

 material have been arranged to run parallel with measurements of the 

 unsatisfied hydration capacity of living cell-masses and of dehydrated 

 tissues. 



The acids and salts have been employed in concentrations mostly 

 within the range of the biological possibilities. It follows, therefore, 

 that these substances have been applied in solutions in which complete 

 or nearly complete dissociation has taken place. 



It was deemed of the greatest importance to traverse a wide field of 

 possibilities, which made the use of simple methods advisable, and 

 solutions have therefore been applied in terms of molar or normal con- 

 centration, and acidity has been determined by titrations. The im- 

 portance of determinations of the acidity, especially of the cell-sap, 

 and its expression in terms of hydrogen- or hydroxyl-ion concentration 

 would be greater in any more critical study of the features of colloidal 

 and protoplasmic action discussed, although it is not to be taken for 

 granted that this is the dominant or determining factor in all cases. 



The use of simple methods has served to reveal the general hydra- 

 tion relations of plant protoplasm, the influence of acidity and tem- 

 perature upon growth and swelling, and to uncover the special effects 

 of the amido-compounds upon hydration and their suggested possi- 

 bilities in affecting growth. 



The significant water-relations of the cell-colloids are not entirely 

 included in direct reactions of the kind mentioned, however. As will 

 be described in Chapter VII, the exigencies of plant life include condi- 

 tions under which dehydration of the plasmatic colloids may reach 

 such a degree that the nature of some of the sugars in growing cells 

 may be affected, and one of these changes is the conversion of poly- 

 saccharids with a low hydration coefficient to pentosans with a high 

 hydration capacity, with the resulting succulency or xerophily of the 

 tissues in which this takes place. 



I am indebted to my colleagues for suggestions and assistance both 

 in the experimentation and in the preparation of the manuscript, 

 especially to Dr. H. A. Spoehr, who has collaborated in previous papers 

 and who has given continued cooperation and valuable advice on 

 various phases of the work presented here. 



Desert Laboratory, D. T. MacDougal. 



Tucson, Arizona, 1919. 



