152 TRANSPIRATION. 



new branches every year, bloom, bear fruit, and 

 supply their wants with nourishment, which could 

 have come to them in no other way than through 

 the wood of their scraped and hollow trunks. 1 



We may also prove that the crude sap rises 

 through the woody tissues by girdling the stem 

 of a tree ; that is, by cutting off a ring of bark 

 without injuring the wood. 2 This does not kill 

 the tree, as may be seen in birch trees which have 

 been girdled for the sake of the bark. But if 

 the wood be cut in a similar ring the tree will 

 soon die. Most of the water in a tree is carried 

 up in the outermost layer of wood, just under 

 the bark. 



When the water reaches the leaves and is carried 

 through the ribs and veins into the cells of the 

 parenchyma of the leaf blade, it cannot escape 

 into the air to any extent through the cell-walls 

 of the epidermis, because their outer walls have 

 been thickened and made waterproof. But the 

 leaves are pierced by openings, connecting with 

 the external air, through which water-vapor can 

 escape and air can enter. These openings are 



1 " Pflanzenleben," Vol. I. p. 252. 



2 See "Lectures on the Physiology of Plants." J. von Sachs. 

 Trans, by H. Marshall Ward. Oxford, 1887. p. 229. 



