XI. THE HYDRATION REACTIONS AND GROWTH OF MESEM- 

 BRYANTHEMUM, HELIANTHUS. AND PHASEOLUS. 



The results obtained by a study of the hydration of the cacti are 

 especially valuable because of the possibility of their correlation with 

 features of varying composition which could be determined by chemical 

 analyses. Measurements of a second type of succulent were sought 

 for the purpose of bringing into relief the possibilities of rapid changes 

 in the water-content of growing and mature organs. A mesembryan- 

 themum {Mesembryanthemum edule) which flourishes in the open 

 at the Coastal Laboratory and in the glass-house at the Desert Labora- 

 tory furnished material suitable for such studies. The leaves attain a 

 length when mature of about 6 to 10 cm. and are triangular in cross- 

 section, the three faces being about 10 to 12 mm. across. Metabolism 

 runs a course in these organs similar to that of the opuntias, as a result 

 of which acid accumulates during the night, and decreases with the 

 disintegrating action of light during the daytime, as illustrated by the 

 data given in table 109. The total daily range in the concentration of 

 the acids is much less than that displayed by the cacti. 



Table 108. — Acidity of juice of leaves of Mesembryanthemum in cubic centimeters 



of N/100 KOH. 



Measurements of the varying diameter of young and of mature 

 leaves indicate that the direct water-loss from the surfaces are so im- 

 portant as to mask the imbibition capacity as affected by acidity and 

 other factors, as will be apparent from auxographic measurements. 



Mention has been made previously of the general similarity of the 

 course of growth of Mesembryanthemum to that of Opuntia. It was 

 possible to go into this matter in greater detail in the experiments 

 described in this volume.^ 



A series of tests was arranged to ascertain the alterations in volume 

 of these leaves both in a mature condition and when in the course of 



* MacDougal and Spoehr. Growth and imbibition. Proc. Amer. Phil. Soc, 56: 314. 1917. 



145 



