126 Hydration and Growth. 



above 2, except in cases in which the rise carried the temperature to 

 30 C. or above and also when the change was as much as 20 C. In 

 the earlier stage of development the rate rose from 0.1 mm. per hour at 

 11 C. to 0.8 mm. per hour at 21 C., and in another leaf from 0.1 mm. 

 to 1.1 mm. per hour in passing from 9 to 20 C. A change from 9 

 to 27 C. resulted in a decreased rate in one case, while in others rais- 

 ing the temperature from 27 or 28 to 33 C. had no effect. 



Falling temperatures were accompanied by reductions, with a usually 

 lower coefficient, it being noticeable that, as exceptions, the rate in 

 one case decreased from 0.7 mm. per hour at 23 C. to 0.15 mm. per 

 hour at 17 C., while in another case the rate decreased from 0.8 mm. 

 per hour at 27 C. to 0.2 mm. at 17 C. This unusual reduction is 

 ascribed to the fact that the door of the thermostat was opened to 

 facilitate the cooling, which resulted in the exposure of the leaves to 

 low relative humidity during the greater part of this period. 



Closing the chamber and continuing the temperature of 16 to 17 C. 

 for 28 hours longer gave a record in which the rate gradually rose from 

 the low figure mentioned to 0.5 and 0.6 mm. per hour, which was nearest 

 the expectancy in comparison with the rate of 0.7 mm. per hour at 

 23 C. which had been displayed in the previous period. 



In the following period a rise of temperature from 17 to 26 or 

 28 C. was accompanied by accelerations of 0.5 to 1.7 mm. and from 

 0.5 to 1.1 mm. per hour. The reversal of the temperatures brought the 

 rates down to 0.6 mm. from 1.7 mm. in a change from 28 to 16 or 17 

 C. and from 1.1 mm. to 0.7 mm. in a change from 26-28 to 17-18 

 C. No other variations were observed in the three weeks in which 

 these organs were under observation which could be ascribed simply 

 to the change of temperature. Usually the change in the thermostatic 

 temperature would be followed within an hour or two by that of the 

 soil around the bulbs and the result would be a gradual adjustment of 

 the rate, increasing or decreasing, with no breaks or erratic features, 

 in the tracings which gave a continuous records of the lengths. 



In conclusion, it is to be pointed out that the foregoing observations 

 show that the hydration capacity of cell-masses may bear some re- 

 lation to the temperature at which they are formed and under which 

 they have functioned for some time, and also that the unsatisfied water 

 capacity of a tissue will be affected by the relation between absorption 

 and water-loss by transpiration. Some gross measurements, which 

 have been made with an accuracy quite adequate for the determination 

 of the point in question, show that plants in the tropics show a rate of 

 growth which may be directly correlated with relative humidity and 

 the transpiration to be inferred. 



Retardation of growth of an organ of a higher plant may be the 

 result of such direct water-loss, or, on the other hand, it may be directly 

 connected with the lessening of the water capacity of colloidal masses 



