Some Hydration Reactions and Growth. 161 



It had been previously concluded that the dominant factor in the 

 varying rate, and in producing shrinkage, was that of modified hydra- 

 tion capacity as dependent chiefly upon the acidity of the sap and the 

 balance between absorption and water-loss by transpiration. The 

 effects of the last-named feature were marked, as the experiments were 

 carried out under conditions of drought approaching the limit of 

 endurance of this plant. Sudden changes in hydrogen-ion concen- 

 tration of the sap of another species of this genus, with other unusual 

 features of the sap as noted by J. Hempel, may be responsible for some 

 of the aberrances shown by this plant.^ While this author gives nitro- 

 gen determinations showing much greater uniformity than in other 

 succulents, this uniformity of total nitrogen may include many changes 

 in amino-groups which might affect the water capacity of the colloids. 

 One species was found especially high in nitrogen. 



The sunflower has been used extensively in studies on growth- 

 rates, and the behavior of young and older intemodes of the same stem 

 of Helianthus furnishes some homologies with the alterations exhibited 

 by the succulents. A stock of Helianthus annuus was grown in the 

 glass-house of the Desert Laboratory in February and March 1918, 

 at which time they showed vigorous and normal development. Dur- 

 ing most of the time in which these observations were made the tem- 

 perature of the air rose to 25° to 31° C. in the glass-house and some 

 slight wilting effects were noticeable in the leaves at midday, a fact 

 included in the records given below, to which are also attached the tem- 

 peratures taken by thermometers thrust into the stems. The young 

 parts in which growing cells constitute the greater part of the mass con- 

 tinue to increase during a period in which the older parts are shrinking. 

 The older parts consist of cylinders of tissue, almost mature, fully 

 saturated, with no continuing increase in hydration capacity; and the 

 growing cells are in the form of an irregular cylindrical shell imder- 

 neath the epidermis, the thickness of which is no more than a small 

 fraction of the entire diameter. Consequently the external measure- 

 ments of the stem are chiefly determined by the changes in the mature 

 cell-masses. 



The preliminary swelling tests were made with sections of the ter- 

 minal intemodes from which tangential slices had been removed, leav- 

 ing them with a thickness of 2.7 mm. Such sections at 14° to 16° C. 

 swelled 7.4 per cent in distilled water, 9.2 per cent in hundredth-normal 

 sodium hydrate, 5.5 per cent in a similar solution of citric acid, and 

 7.4 per cent in a similar solution of potassium nitrate. These results 

 are fairly representative of this type of plants. 



The preceding series was made up before the daily shortage of water 

 in the terminal parts of the stems had been detected. A second set of 



^ Hempel, J. Buffer processes in the metabolism of succulent plants. Compt. Rend. d. Trav. 

 d. Lab. d. Carlsberg, 13. 1917. See pp. 4&-51. 



