234 LECTURE XIV. 



Stem and in the venation of the leaves, serves at the same time to deliver the water 

 in a proper manner to the badly conducting masses of parenchyma. The lignified 

 supporling tissues in slender upright stems are therefore brought close to the 

 periphery, not only because they thus best promote elasticity, &c., but also because the 

 loss of water at the surface of the epidermis can thus be most easily made good. 



FIG. J96.— Lignified, water-conducting spiral cells and fibres in tlie ven^onoflh^leaioiAntkyltisvidiieyaria. 

 Tlie enclosed interspaces or areolae are filled up with transpiratory assimilating tissue. 



The same consideration shows that the venation of the leaves serves at the same 

 time their elasticity and rigidity, and the appropriate supply of water. 



The wood is thus endowed with the specific property of allowing rapid movements 

 of water to take place in it : it is therefore of interest to know how rapidly a particle 

 of water moves in the wood of the stem under particularly favourable circumstances 

 — that is, during very active transpiration from the leaves. Numerous researches 

 which I have made in this direction with various kinds of plants, show that 



