THE WATER-BALANCE OF SUCCULENT PLANTS. 



By D. T. MacDottgal and E. S. Spalding. 



INTRODUCTION. 

 GENERAL ASPECT OF THE PROBLEM. 



The activities of the seed-plant make necessary a movement of solu- 

 tions from the surface intakes of roots or other absorbing- organs in con- 

 tact with the medium or substratum to various tissues, the residue of 

 liquid and content finally reaching the excretory or transpiratory mem- 

 branes of the leaves, stems, or other aerial members. This water-service 

 involves a supply of available moisture in the substratum or medium, 

 absorbing organs, whose tissues sustain a higher osmotic activity than 

 the substratum, conducting tracts of adequate capacity, and transpiratory 

 organs, which in response to the evaporating action of the air throw off the 

 water of the solutions entering the roots, in the form of vapor, at a rate 

 variously modified by the purely physiological activity of the protoplasts. 



The individual and contributory action of all of these factors is greatly 

 influenced by many internal and external conditions, physiologic, morpho- 

 genic, and physical. The relation of the plant to the moisture-supply and 

 to the evaporating capacity of the air constitutes a limiting factor of great 

 importance in determining distribution and habitat selection. 



It is evident that the moisture relation is one of the most intricate of 

 those entering into the environmental complex, and that it is correspond- 

 ingly difficult of analysis. Partly as a result of this complication, the 

 mechanism of the ascent of sap in the trees and the value of transpiration 

 as a function have been singled out for an amount of attention far beyond 

 their relative importance. Both questions have now come to be discussed 

 in a purely academic manner, and with but little actual progress. Mean- 

 while, actual experimentation continues to yield profitable results in the 

 hands of the few workers taking up the subject by the proper methods. 

 The present paper is chiefly concerned with the results of observations 

 upon plants characteristic of arid regions, and which consequently take up 

 solutions from the substratum under well-defined conditions widely different 

 from those to which the plant in moist temperate regions is subject. 



The soil undergoes daily variations in temperature of wide range, and 

 the percentage of soil-moisture sufficient to yield a supply to the plant is 

 afforded only during very limited periods. The plant may carry on the 



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