146 



Hydration and Growth. 



growth. Their regular surfaces made it possible to place one side 

 snugly on a small wooden block, and to bring the cork-tipped vertical 

 auxograph lever in bearing with the uppermost angle of the leaf. The 

 stem was held firmly a few centimeters from the base of the leaf, which 

 was exposed to no disturbing conditions (fig. 33). First, the diurnal 

 variations of a mature leaf were 

 followed without any attempt being 

 made to equalize temperatures, 

 which were taken by a thermometer 

 with a thin bulb thrust into a 

 second leaf and allowed to remain 

 there. The readings were as low 

 as 10 C. at daybreak and as high 

 as 31 C. at midday. The varia- 

 tions in volume amplified 45 times 

 shown in figure 34. The record 

 beginning at noon on February 14, 

 with the temperature of the leaves 

 at 25 C., was at a juncture when 

 shrinkage set in, lessening the 

 thickness of the leaf about 1 mm., 

 or 10 per cent of its turgid thick- 

 ness before 5 h 30 m p. m., at which 

 time, with the temperature still at a 

 high point (27 C.), the shrinkage 

 came to an end and enlargement 

 began, which continued through the 

 night, so that at 8 o'clock the follow- 

 ing morning the leaf was actually 

 thicker than on the preceding day. 

 The temperature on this day rose 

 to 31 C. and the shrinkage exceeded 

 that of the preceding day, amount- 

 ing to about 13 per cent of the 

 turgid thickness of the leaf. Swell- 

 ing began again in the evening, 

 which restored the leaf to about its 

 original dimensions 48 hours after 

 the beginning of the record. 



The variations appeared to run parallel in part only to those already 

 described in detail f or Opuntia, and to the extent that the daily variations 

 take the form of alternate shrinking and enlargement with the increase 

 in excess of the loss. The chief features of growth may be illustrated by 

 the record of one of a pair of leaves which had attained about two-thirds 

 of the full size. This was put in bearing with the auxograph in the 



FIG. 33. Detail of arrangement of auxo- 

 graph to record variations in thickness of 

 leaf of Mesembryanthemum. Leaf in a 

 horizontal position resting on a wooden 

 support cut away below to make place for 

 the younger terminal leaves. 



