LIMITING FACTORS TO GROWTH MEASUREMENTS. 325 



that the curves of temperature between 6 p.m. and 6 a.m. are 

 markedly parallel with the curves of growth for the same 

 period. We seem here, therefore, to have a case in which one 

 physical factor, humidity of air, regulates the growth during 

 the day, and another, temperature, at night. I think the 

 theory of Limiting Factors is fairly applicable to this case. 

 If we suppose that during the day the transpiration of the 

 adult culms is so great as to draw off the supply of water from 

 the growing culms, we get the reason why in the daytime 

 the growth shows a correlation with the humidity of the 

 atmosphere. After 6.30 p.m. darkness and the saturation 

 of the atmosphere produce a set of conditions in which trans- 

 piration from the adult culms is practically nil. The supply 

 of water to the growing culms may then be supposed to be 

 in excess, so that the temperature, which is considerably lower 

 than this species ever naturally experiences, becomes the 

 limiting factor and continues so until with the next morning 

 the light and the decrease of humidity again bring about 

 conditions in which the water supply is the limiting factor. 



Humidity only indirectly a Limiting Factor. 

 If we are to apply this theory to our observations of Den- 

 drocalamus at Hakgala we must at once recognise that the 

 Humidity of the Atmosphere is only indirectly the controlling 

 factor in the growth at Peradeniya or Anuradhapura or in the 

 daytime at Hakgala. The young culms are on the same 

 rhizome as a number of adult leaf -bearing culms, and the 

 supply of water to the young ones depends almost entirely on 

 the amount of water drawn off by the transpiration of the 

 adult culms. If transpiration is vigorous the water supply 

 to the growing culms is small, and vice versa. Now the trans- 

 piration of the adult culms, depending as it does upon the 

 opening or closing of the stomata, is not regulated merely by 

 the one factor of the humidity of the atmosphere, but by the 

 light also. Light is generally held to cause an opening of the 

 stomata and therefore an increase of transpiration. 



