LIMITING FACTORS TO GROWTH MEASUREMENTS. 33] 



given for the day, the highest humidity is usually found either 

 late in the evening or early in the morning, thus pointing to the 

 supposition that the humidity in the night was higher than 

 in the day. 



This being the case a full curve of humidity would probably 

 show a fairly close relationship to the curve of growth and the 

 possibility of humidity being the limiting factor throughout 

 is by no means excluded. The Psychrometer figures* during 

 the day do not fully account for the exact position of the maxima 

 and minima of growth, yet the general explanation relating to 

 humidity here suggested seems preferable to the supposition of 

 Sachs that the retarding light effect of the day is stored up in 

 such a way that its influence is not removed when night comes 

 on, but persists until early the following morning. 



The results of F. Darwin's measurements on the growth of 

 the fruit of Cucurbita (5) show very clearly the relation between 

 growth and the water supply, the latter depending largely on 

 the amount transpired by the leaves. The very complete set 

 of tables of results and curves of growth led me to search here 

 for a similar case to my measurements of Dendrocalamus at 

 Hakgala, that is to say, a case in which moisture being the 

 limiting factor as a rule, the temperature became limiting 

 whenever the moisture was in good supply. I looked therefore 

 for continuous periods of high humidity to see if during these 

 the growth curve resembled the curve of temperature and not 

 the curve of humidity. 



No decisive case was found, though in some periods such an 

 explanation would do no violence to the figures given. 



It was impossible to find a period of sufficient length during 

 which one could be certain that the humidity was high enough 

 to leave temperature as the limiting factor. The possibility of 

 demonstrating that temperature was limiting at night in the 

 case of Dendrocalamus at Hakgala depends, it must be remem- 

 bered, upon the fact that for 12 continuous hours of darkness 



* As Look points out Sachs' Psychrometer figures are not very reliable. 



