LIMITING FACTORS TO GROWTH MEASUREMENTS. 331 



of growth also shows, a considerable falling off, and when the 

 temperature is nearly uniform the growth curve is also nearly, 

 uniform, seems to me to be fully established. It may be pointed 

 out here, as in the case of the Agaves, that the temperature 

 observations are liable to an error of 1° F. and that the mean 

 for the periods is obtained in a way which can only give, in the 

 absence of intermediate readings, roughly approximate figures. 

 Further, if we suppose (as we did for Agave) that the true 

 controlling factor is the internal temperature of the growing 

 culm itself and not the temperature of the surrounding air 

 the two or three seemingly aberrant results may be to some 

 extent accounted for. 



Thus, the general correlation between the fall of air 

 temperature and the fall of growth rate through the night is 

 unmistakable and fully supports our hypothesis. 



The above observations were made on two different culms, 

 the figures for the three nights, July 9, 10, and 11, referring to 

 one culm and those for the other eleven nights referring to 

 another culm in a different clump. The curve for July 10-12 

 on Plate 23 is typical of the nights of nearly uniform tem- 

 perature accompanied by nearly uniform growth. 



2. Day Growth with rain at Hakgala. 



Since the temperature is as a rule considerably higher during 

 the day than the night, we should expect if the supply of water 

 could be maintained in excess that faster growth would be 

 observed in the daytime. Conditions under which this could 

 be tested rarely occurred, as in wet weather the difference 

 between day and night temperature is usually very small, 

 while the weather which shows a considerable contrast between 

 day and night temperature is dry and the days usually without 

 rain. It is significant in this connection to notice that when 

 two hours of more or less continuous rain did occur during the 

 day after sunny weather, with fairly high temperatures, the 

 growth recorded was more rapid than occurred at any time. 



