172 



THE LIFE OF THE PLANT 



in the form of the finest threads. This phenomenon 

 is seen best if the microscopic preparation is lighted 

 from above instead of in the usual way from below ; 

 then we clearly see in the vessels bright threads of 

 mercury like that seen in the capillary tube of a 

 thermometer (fig. 49, I). 1 Let us remember that 

 mercury does not rise by itself as, for instance, water 

 does in a capillary tube, but that on the contrary 



it can only be forced into 

 such tubes by pressure 

 the narrower the tubes the 

 greater must the pressure 

 be. But the diameter of 

 plant vessels is much nar- 

 rower than the diameter of 

 h~ the capillary tubes with 



which experiments in physics 

 are generally made. Hence 

 we can measure approxi- 

 mately the degree of its 

 rarefaction that brings about 

 this aspiration of mercury. 

 Two questions naturally arise : 

 p IG 49 why is no equilibrium estab- 



lished between the rarefied 



gases in the vessels and the external atmosphere ? 

 and, what is the reason of that rarefaction ? The first 

 question is answered very simply : the air inside the 

 vessels is separated from the external parts of the 

 plant, containing air at the general atmospheric pres- 

 sure, by a layer of impenetrable tissue, which entirely 

 isolates it from the external atmosphere. But when- 

 ever the internal parts of an organism come into con- 



1 Microscopes for handing round to the audience at lectures are specially 

 convenient and have been much used of late years. They are provided 

 with concave metallic mirrors which concentrate the light upon the upper 

 surface of the object. 



