156 



Hydration and Growth. 



It will be seen by reference to the records of growth cited in table 116 

 that the higher temperature lies above the point at which the most 

 rapid elongation or thickening takes place, a matter which might be 

 due to excessive water-loss or to the action of residual acids at high 

 temperatiires. 



c^:GS 



Beans nearly mature but stiU in the process of enlargement were 

 removed from green pods, the ends of the cotyledons cut away, and 

 then a slice removing the hypocotyl; the remainder of the cotyledons 

 came away free from the outer coating or mem- 

 brane. The average diameter of trios of such sec- 

 tions was 3 to 3.2 mm., and their sweUing was as 

 given in table 117. 



The amount of hydration was less at the higher 

 temperature in distilled water, suggesting that the 

 point of maximum imbibition or swelling Ues be- 

 low 38° C. In the presence of acid the amount of 

 water absorbed is distinctly less than at 18° C. 

 These data being available, attention may now be 

 profitably turned to the features of enlargement of 

 the pods. 



Preparations were made by which delicately 

 weighted auxographs recorded the variations in 

 thickness of pods in the stage when about half the 

 final length had been reached. The end of the 

 bearing-lever rested over an enlarging bean, and 

 variation during a week is shown in figure 40. 



The localization of growth had been previously 

 determined by the well-known expedient of mark- 

 ing a young pod which was in the stage of initial 

 growth of the young beans into four 1-centimeter 

 intervals (fig. 41). Swelling tests had been made 

 of the pods in this stage (table 116). Ten days 

 later the basal and apical intervals had increased 

 to 2.5 cm., whUe the other two had each elongated 

 to 4 cm. All measureinents of variations in thick- 

 ness were made in this median region of maximum elongation (fig. 42). 

 , Growth both in length and thickness of the young pods was at the 

 lowest rate at night, during which period the temperature was at 15° C. 



Fig. 40.-.— Arrangement of 

 vertical lever of auxo- 

 graph making record 

 of variations in thick- 

 ness of growing bean 

 and pod. 



