370 



MacDOUGAL— HYDRATION AND GROWTH. 



20 to 25° C. at 3 P.M. was followed by a lessened rate of growth, 

 and on the cloudy days was uniformly high. Similar effects were 

 exhibited by a small fruit of a potato in a greenhouse at Tucson in 

 May, 1918. 



The water deficit of the stems as measured by swelling includes 

 that of the entire structure. The fruits however receive their supply 

 through special conduits which sustain only a mechanical relation to 

 the other parts of the stem which may be active in its swelling. 

 Such non-conducting tissues of course draw their supply from this 

 system of conduits also, but it is highly probable that the dispropor- 

 tion between the water content of the fruit and of the tracts in the 

 stem from which it receives its supply is not so great as might be 

 indicated by the measurements given. The hydration capacity of 

 the fruits would be the resultant of many factors including the 

 pentosan-protein ratio, the hydrogen ion concentration, the action of 

 salts and the effect of the amino-compounds. 



NojEn wff RSn Mjf N$n t^i nJoo fS NSoii fSf NoEn m! N3on W 



Fig. 3. Tracings of auxographic records of Opuntia discata during two 

 weeks of secondary growth. Downward course of the pen denotes enlarge- 

 ment amplified 45 times. The scale is in centimeters. The shrinkage at night 

 is greater in some instances than the enlargement of the preceding day. 



The balance between water-loss and the gain by absorption is so 

 delicate in the fruits the action of which was measured, and in many 

 stems that increased humidity may be followed by accelerated 

 growth, while a rise of ten to fifteen degrees in air temperature may 

 check enlargement by increasing water-loss and it is also taken into 

 account that the consequent rise in temperature of the growing stem 

 may actually lessen the water-holding capacity of the biocolloids 

 which make up living matter. 



