150 Hydration and Growth. 



pronounced in the layers which had been removed. The mature 

 leaves were distinctly flabby to the touch and the external layers were 

 in a state of partial collapse during the midday period. 



Segments of young leaves in the stage in which these organs con- 

 tinued elongation and increase during the entire day were now taken 

 for comparison. Their thickness was a little more than half that of 

 the mature leaves. The trio of sections taken from leaves at 8 a. m., 

 when the highest turgidity prevailed, showed a swelling of 13.6 per cent, 

 while those taken at midday swelled 16.7 per cent at 14° to 20° C. 

 The difference in the two cases is much less proportionately than that 

 which is set up in mature leaves and fully accounts for the shrinkage 

 of the older organs. Beyond this the auxographic tracings made obvi- 

 ous another difference between the young and mature leaves. The 

 total swelling in the mature leaves was reached in 6 or 8 hours, most 

 of which developed within 2 hours of immersion. The swelUng of 

 young leaves was much more gradual, and the rate was less rapid at 

 first and then decreased much more gradually and had not actually 

 ceased at the end of 20 hours. The material in the young leaves was 

 not only alive, but in a growing condition; consequently new colloidal 

 material in the process of aggregation would provide a continuing 

 source of hydration capacity. 



Confirmatory evidence consists in the fact that when two pairs of 

 leaves are put in bearing with auxographs, the one exposed to the sun 

 soon reaches the stage where the water-loss at midday is equivalent 

 to growth and neutralizes it on the record, while the pair of leaves 

 of the same age shaded from the sun continues elongation scarcely 

 checked or retarded during the same period. Now, if the preparation 

 in the sunny location experiences a cloudy day or is shaded, it too 

 continues growth during the entire day in a manner which shows con- 

 clusively that the daily retardation is in the main due to excessive 

 water-loss in the case of the Mesembryanthemum. A similar action by 

 the cacti has already been discussed in Chapter X. 



The practice of testing the swelling of dried sections for comparison 

 with that of living material was followed as in Opuntia. Sections of 

 living leaves about 5 mm. in thickness were prepared as above, and 

 these were placed between folds of filter-paper and weighted only 

 sufficiently to prevent warping and curling during desiccation, which 

 continued for a week. Their final thickness was about 0.25 mm. and 

 their swelling, unlike that of the segments of Opuntia, did not come 

 back to the approximate size of the fresh material. The final measure- 

 ments in percentages of the dry thickness were as follows: 



Table 112.* 



p. ct. 



Distilled water 100 



Citric acid, 0.01 N 120 



Sodium hydroxid, 0.01 M 100 



