THE STEM 171 



young leaves evaporate more quickly than old ones of 

 the same plant, and this fact affords a partial explanation 

 of why the nutrient sap flows precisely to these young 

 growing organs. 



Having learnt what a considerable quantity of water 

 is evaporated by the leaves, we can return to the in- 

 vestigation of the actual mechanism of this movement 

 in the stem. 



This question has attracted special notice of late, 

 and yet it cannot be said to have been settled quite 

 satisfactorily. It is true that there are many explana- 

 tions of the phenomenon, but their very abundance 

 proves that none of them is completely satisfactory. 

 Let us then dwell only on facts easily verified by 

 experiment. To begin with, it was necessary to decide 

 what course the stream of water takes up the 

 stem : does it pass by way of the cavities, or within 

 the walls of the vessels ? Contrary to the first 

 most natural supposition that the sap circulates in the 

 cavities of the vessels, since this would appear to be 

 their simplest course and since these cavities form 

 continuous channels through the plant, it has been 

 pointed out that vessels are not generally filled with 

 liquid, but contain bubbles of air alternating with 

 columns of liquid. Yet this very presence of air, which 

 at first was an objection to the theory that water 

 circulated through the vessels, is now taken as a key to 

 the explanation of the phenomenon. It happens that 

 this air generally exists in a very rarefied state, and 

 that owing to this circumstance every vessel acts as a 

 pump. We can demonstrate this fact by the follow- 

 ing simple experiment : a stem of any kind is bent 

 down into a vessel containing mercury until one part 

 of it is sunk under the mercury, and it is then cut 

 across under the mercury. If we make longitudinal 

 sections of the same stem later on, we shall see that 

 mercury has penetrated into the cavities of the vessels 



