172 



THE LIFE OF THE PLANT 



I 







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).i 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 

 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 : 

 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- 



E 



Fig. 49. 



' 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. 



