148 



Hydration and Growth. 



afterward, beginning actual shrinkage by 9 a. m. Growth of the leaf 

 of middle age slows down with the low temperature of daybreak, but 

 accelerates at 9 a. m., at the time the older leaf begins shrinking, at 

 17° C. The youngest pair of leaves grows with a high rate all night, 

 with a perceptible slowing at daybreak, but it also accelerates at 9 a. m. 

 with the rising temperature, at the time the oldest leaves are shrinking 

 (fig. 37). 



I2p.m 



12 p.m. 



I2p.m. 



3I*C. 



Fig. 36. — A, auxographio record of elongation of pair of leaves 2 cm. long during a 24-boui 

 period; temperatures of 14" to 25° C. Retardation during period of highest temperature is 

 illustrated. X20. B, action of same leaves in shade. 



The general features of growth imder the usual varjdng conditions 

 of alternating dayUght-high temperature and night-low temperature 

 complexes being determined, it became necessary to test the sweUing 

 of the leaves, as had been done with the joints oi Opuntia to ascertain 

 their unsatisfied hydration capacity. 



Preparations for testing the sweU- 

 ing of living material of leaves were 

 made by placing these triangular 

 organs on a flat siuiace alongside a 

 guide of 5 mm. in thickness. A 

 razor sUd along this slices away the 

 uppermost angle, leaving a trun- 

 cated section 5 mm. in thickness, in 

 which the central fibrovascular 

 tissue could be seen through the 

 translucent parenchymatous tissue. 

 Segments about 1 cm. long were 

 taken, to the exclusion of the basal and apical parts of the leaf. It is 

 to be noted that active enlargement is usually in progress morning 

 and evening in young leaves, while mature leaves are enlarging ia the 

 morning but shrinking in the afternoon and night. The results of the 

 hydrations are shown in table 110. 



In the above tests the amoimt of swelling was most in distilled water, 

 less in hydroxid, and least in acid, in mature leaves taken in the even- 



ts 



55 



Fig. 37. — ^Variations in thickness of a mature 

 leaf of MesembryarUhemum. X46. Tem- 

 peratures taken from a similar leaf with 

 a mercuiial thermometer. 



