352 



THE EFFECT OF WATER 



[Ch. XII 



It appeared that as the atmospheric moisture increased, a 

 considerable increase in the cross-section of the stem followed ; 

 and as it diminished, the size of the stem diminished likewise. 

 The results are graphically given in Fig. 95. 



Fig. 95. — Curve of growth in thickness of a Datura stem, M-M, correlated with 

 variations in relative humidity, H-H, the temperature T-T remaining nearly uni- 

 form. The part of the curve falling helow 0-0 indicates loss of thickness below 

 the normal. The ahscissEe represent hours. (From Beinkb, 76.) 



These experiments have been repeated by Francis Daewijst 

 ('93) upon the fruit of a gourd, Cucurbita, whose growth is 

 little influenced by variations in temperature. He determined, 

 by means of a delicate micrometer apparatus, the average incre- 

 ment in microns per minute ; and, at the same time, by means 

 of a dewpoint thermometer, the moisture of the air. The 

 relation between rate of growth and psychometric readings is 

 best shown graphically, as in the curves of Fig. 96. The 



Fig. 96. — Curve of diameter of fruit of Cucurbita (full line) correlated with varia- 

 tion in humidity (broken line). The abscissae represent hours; the ordinates 

 represent growth in /i per minute (numbers on the left) and per cents of humidity 

 (numbers on the right). (From Darwin, '93). 



