LIMITING FACTORS TO GROWTH MEASUREMENTS. 329 



The same reasoning would apply to the observations at 

 Anuradhapura, except that here the exudation of water usually 

 occurs later still as the humidity is as a rule lower than at 

 Peradeniya. At Anuradhapura the average humidity for 

 June and July is 77 per cent. , while at Peradeniya for the same 

 months it is 81 per cent, and 82 per cent, respectively. 



We do not get therefore suitable conditions for a sufficient 

 length of time to exhibit the limiting effect of temperature 

 when water supply is in excess of requirement. 



Summary of General Explanation of Facts. 



To sum up, the general explanation which I offer of the 

 observations on Dendrocalamus at the three places is, that 

 two factors of the physical environment chiefly affected the 

 rate of growth of the young culms: (1) the temperature; (2) 

 the supply of water to the growing culms. This second factor 

 being intimately connected with the amount of water drawn 

 off by transpiration by the adult culms which are on the same 

 rhizome, depends in its turn on two factors : (1) the humidity of 

 the atmosphere ; (2) the intensity of the light. 



During the day at Hakgala, and during the day and also a 

 varying part of the night at Peradeniya and Anuradhapura, the 

 conditions are such that the supply of water is the limiting 

 factor in the growth of the culms, the temperature being high 

 enough to allow of greater growth than actually takes place. 



During the whole night at Hakgala, and during a varying 

 part of the night at Peradeniya and Anuradhapura, the mois- 

 ture supply is in excess, and temperature is the limiting factor. 



Further Tests of the Theory. 



1. Night Growth at Hakgala. 



If this is the true explanation of the facts, we should expect 

 that the falling off of growth at Hakgala during the night 

 would vary in amount according, as the fall of temperature 

 through the night was greater or smaller, i.e. , the curve of night 



9(8)06 (30) 



