VII. FLUCTUATING OR ALTERNATING HYDRATION EFFECTS. 

 BASIS OF XEROPHILY AND SUCCULENCE. 



The experiments described in the previous chapters have dealt 

 chiefly with sections of colloidal material artificially compounded to 

 represent the materials and conditions which affect hydration in plants. 

 Measurements of the swelling of dried sections of this material have 

 been used as a basis for comparison with the action of slices or sections 

 of plant cell-masses similarly dehydrated or in living condition. 



No estimate of results of this kind will be valid and no perspective 

 of their bearing will be correct which does not take into account 

 the fact that the growing parts of the higher plants contain 90 per 

 cent or more water and that the colloids of the protoplast, the action 

 of which makes for the distension or enlargement of growth, are even 

 more highly hydrated. These gels also invariably contain the culture 

 salts, in combination or simply adsorbed, and are inevitably in a con- 

 dition of acidity resulting from their carbohydrate metabolism. 



The elementary facts obtained by the experiments described in the 

 foregoing pages made it possible to plan a series of treatments of 

 hydrating material in which the previous experience of biocolloids 

 would be apprehended in swellings in salts, acids, etc., in a sequence of 

 interest to the physiologist, and to obtain additive, alternating, or 

 superposed effects. Variations in the carbohydrate and proteinaceous 

 substances of a colloidal mixture may not be readily produced to 

 simulate the changes which form the basis of some of the most funda- 

 mentally important phenomena of the cell. The worker must approach 

 this phase of the subject by studying the reactions of separate masses 

 or lots of material compounded to represent various stages in the 

 condition of the protoplasm. Something of this kind has been done 

 in the measurement of the reactions of biocolloids of several kinds. It 

 is of course impossible to imitate any of the important metabolic 

 processes which make cell-colloids continuously acting machines, al- 

 though no hint has been found of any source of energy or directive 

 action outside of surface tension and chemical action. 



In the subjection of colloidal masses to the action of hydrating 

 solutions, as described in the following pages, it is obvious that the 

 soluble constituents of the sections would be partially and unequally 

 removed with every change in the solutions in which they were sub- 

 merged, and while the fact that the colloidal mass changed its com- 

 position continuously during the immersion in the various reagents, 

 yet it can not be said that these alterations were identical with those 

 of the growing cell. 1 Some highly suggestive results or situations were 

 produced, however, by the replacements of hydrating solutions, as 

 described in following pages. 



NOTE. All measurements of swelling and shrinkage are given in terms of original or dried 

 thickness of sections. AUTHOR. 



1 MacDougal and Spoehr. The effects of acids and salts on biocolloids. Science, 46 : 269. 1917. 

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