LIMITING FACTORS TO GROWTH MEASUREMENTS. 357 



temperature varies so little as in June or July in Ceylon the 

 soil temperature varies less still and the readings of soil temper- 

 ature at a depth of one foot taken at 9.30 a.m. and 3.30 p.m. 

 very rarely show a difference of more than 3° F at Peradeniya. 

 None of the phenomena of growth observed in my work could 

 with certainty be ascribed to so uniform a factor. Where 

 however the soil temperature varies considerably it should 

 certainly be taken when possible. The loss of water from 

 leaves and stem depends on the humidity of the atmosphere 

 and upon the intensity of the sun's radiation. 



Other cases such as Dendrocalamus are by no means so 

 simple as that of Agave, for since the supply of water to the 

 growing organ depends on the amount required by the other 

 organs of the plant, conditions become very important which 

 do not directly affect the growing organ itself. Another 

 instance of this is the case of Cucurbita fruit in which the 

 supply of water to the fruit is conditioned by the amount 

 required by the leaves for transpiration so that the growth of 

 the fruit is affected by every change which alters the trans- 

 piration of the leaves. 



Again the case of Vitis Lindeni shows us another possibility, 

 for here we have a case of which one possible explanation is 

 that the number of the growing organs decides whether the 

 water supply shall be limiting or not. If with a given number 

 of absorbing roots the supply of water is sufficient for a small 

 number of growing roots, it may be insufficient for a larger 

 number. This is on the assumption that the number and 

 activity of the absorbing roots are not adjusted to the number 

 of growing organs. It seems quite probable however, and 

 investigations on this point are being carried on here, that the 

 total water absorbing power of the roots of a given plant is 

 adjusted to the demand made by the growing organs. If this 

 is so the above explanation of V. Lindeni does not hold good, 

 but whether it is so or not, we may still keep in mind the above 

 explanation as one of the possibilities with which we have to 

 deal in tin- application of our theory. The supply of water \>> 



