226 STRUCTURAL BOTANY 



from the leaves. If an actively transpiring plant be 

 cut off just above the ground and placed in water, 

 the amount which its leaves give off as vapour may 

 be more than ten times as much as that which is 

 pumped up from the stump by root -pressure during 

 the same time. 



That the water ascends the stem through the wood 

 can be well shown in any dicotyledonous tree in which 

 the wood forms a continuous ring. Indeed, the ex- 

 periment might be quite well carried out on our 

 dicotyledonous type, the Wallflower, but a larger 

 woody plant is more convenient for the purpose. 

 A ring of tissue is removed from the stem, the part 

 stripped off reaching inwards as far as the cambium, 

 so that the whole of the cortex and phloem are taken 

 away. The wound is covered up with damp cotton- 

 wool, or protected in some other way against evapora- 

 tion. No withering of the leaves takes place, they 

 remain indefinitely fresh and turgid as before ; and if 

 we measure the transpiration, the amount of watery 

 vapour given off is found to be undiminished. Hence 

 it is evident that the current of water goes up just 

 as well when the phloem and cortex are interrupted 

 as it does when they are complete. Only the wood 

 and pith remain for it to pass through. It makes no 

 difference to the result of our experiment if, as is 

 usually the case, the pith is completely dried up and 

 quite incapable of conducting water. Hence we can 

 only conclude that the whole ascending current of sap 

 passes through the wood. 



In many trees, as for example the Beech, the whole 

 thickness of wood continues to conduct water, even in 



