LIMITING FACTORS TO GROWTH MEASUREMENTS. .'i27 



humidity is probably only acting through the epidermis of 

 the leaf. 



Close examination of the curve for Anuradhapura seems to 

 me to lend support to this view. One of the striking features 

 of this curve is that there is a very marked change in rate of 

 growth between 4 p.m. and 8 p.m. , which does not seem to be at 

 all adequately accounted for by the change in humidity. A 

 corresponding rapid change in the opposite direction the 

 following morning after 6 a.m. is also greater than the general 

 correspondence between the curves of growth and humidity 

 would lead us to expect. It seems to me reasonable to suppose 

 that there we can trace the influence of the change from light 

 to darkness and the reverse. 



In this case the day was almost uniformly sunny and the 

 light intense, so that the change of light intensity at sunset 

 and sunrise was very marked. That this marked and (as 

 always in the tropics) fairly rapid change in the intensity of 

 light caused a considerable change in the transpiration of the 

 adult culms and thus indirectly affected the growth of the 

 young culms seems to supply an explanation for the very 

 considerable changes which would otherwise be inadequately 

 accounted for. 



It is also easier to see on this view why the daily growth of 

 Dendrocalamus at Peradeniya should only once in the total 

 series of measurements made by Lock (12) and myself have 

 exceeded the growth at night. When the day (as happens 

 fairly frequently) is one of continuous high humidity with 

 continuous or intermittent rain, there seems no reason to expect 

 on any hypothesis that the day growth should not equal or 

 exceed the growth at night. If, however, we consider that 

 even when the humidity is high during the day, the light is 

 always far more intense than at night, we see that the transpi- 

 ration is likely to be greater in the day even when the humi- 

 dity during day and night are almost equal. Thus, even on 

 days of high humidity we should not expect the growth 

 during the day to equal that during the night. It seems 



