136 



Hydration and Growth. 



of 5.2 mm. daily at 16° to 18° C. showed a rate of 11 to 17 mm. daily 

 at 30° to 32° C. The rate of growth would be one which would be 

 accepted as being in general conformity with the van't HofI formula of 

 chemical reaction, while as a matter of fact it is not widely different from 

 the capacity for imbibition under the influence of equivalent tempera- 

 tures. Experimental tests have already been described in which the 

 upper limits of growth and the behavior of etiolated andgreen stems of 

 Opuntia between 46° and 51.5° C. are not widely different. (Fig. 24.)^ 

 The rapid and wide variations at these high critical temperatures 

 are to be contrasted with the steady rate of a growing green joint main- 

 tained at 30° C. for 38 hours. The illumination in the daylight period 

 apparently did not affect the rate of 0.07 mm. per hour. 



12 p.m. m 



Fig. 25. — Tracing of auxographic record of variations in thickness of joint of Opuntia approach- 

 ing maturity. Upward movement of pen denotes increase in thickness, X45. Measurements 

 made from place between areoles near base of joint. Scale ruled to 5 mm. and 12-hour 

 intervals. It is to be noted that the hour is set forward on the summer schedule. 



In addition to the great number of records of the variation of joints 

 of Opuntia as to length, a few days' measurements of the thickness of a 

 maturing joint were made in April 1918 in order to ascertain whether 

 or not increases and shrinkages took place in all dimensions at the 

 same time. The auxographic tracing in figure 25 gives the daily 

 variation in thickness near the base of a maturing joint, which are seen 

 to be correspondent to those in length. It is to be noted that the 

 instrument was necessarily adjusted so that increase was denoted by 

 the upward movement of the pen, in a manner opposite to that in 

 nearly all the other records. Later the bearing-lever of the instru- 

 ment was adjusted to a place near the apex of the joint and the shrink- 

 age, which was now pronounced, was seen to set in about 9 or 10 a. m. 

 and continue until sunset, at which time thickening began and lasted 

 for 3 or 4 hours, after which but Uttle change took place until the rising 

 temperature of the following day was encountered. 



The singular retardations and variations in growth which are highly 

 characteristic of Opuntia, together with its unusual features of trans- 

 piration and its readily measurable imbibition, offer unusual oppor- 

 tunities for the examination of certain agencies affecting growth, 

 especially as the action of some of these factors is so plainly discernible 

 in the variations in volume of mature organs. 



* MacDougal and Spoehr. Growth and imbibition. Proc. Amer. Phil. Soc, 66: p. 308. 1917. 



