356 SMITH LPPLIOATIOH 0» THE THEORY OF 



of course the internal temperature is the one at winch 



all the organic chemical changes of the growing shoot take 

 placn. 



Water supply. 



It is no less obvious that the supply of water to the growing 

 shoot is an import am tact or. When we consider how high a 

 percentage of the weight of such a shoot is water we at once 

 realise how necessary it is for water to be in good supply. This 

 factor is by no means so simple as the last in its action. It often 

 depends, as in the case of Dendrocalamus, on two or more other 

 factors and varies in many cases with the organisation and 

 habit of the plant as a whole. 



In the case of such plants as Agave where the growing shoot 

 is the only one on the plant the case of water Supply is simple. 

 The amount available for the growing shoot is evidently the 

 amounl taken up by the roots, minus the amount lost by 

 evaporation or transpiration from the shoot and leaves. 

 The leaves themselves no doubt supply water to the shoot 

 since the reserve substances conveyed from them to it are in 

 solution, but we may confine ourselves to the root supply. 

 minus the loss by transpiration. The supply of water taken 



up by the roots depends evidently on the extent and vigour of 

 the rout system, and any defert in this respeel may Cause a 

 limiting wain supply. This was probably the ease with I )e 



Varies 1 Agave which, he. says, had no roots but absorbed watei 

 by a porous dead mass covering the living cool stuck. Given 

 hours ei a vigorous root system the absorption chiefly depends, 

 i - long as the water content of the soil is sufficiently high, on 

 the soil temperature. We know that a oold soil containing 

 plenty of water may yet be physiologically drj . I he absorption 

 of water being limited hy the low temperature, h is this faot 

 which makes it necessary in obtaining measurements to whioh 

 our theory can be applied, to take the soil temperature when 

 ible. The records for soil temperatures in Ceylon 

 show that these are remarkably uniform. Where the an 



