38 PLANTS 



third, the true vessels, which originate from rows of cells whose 

 dividing walls disappear, leaving a continuous passage from cell 

 to cell. The walls of the true vessels are also thickened in 

 spiral lines and otherwise as in the tracheides. The tracheides 

 and vessels lie on the side of the vascular bundle next the pith, 

 while the bast and sieve vessels lie next the surface of the stem. 



75. The bast and wood portions of the vascular bundles are 

 separated by a zone of very thin-walled cells. This is the 

 cambium, the region in which the new cells are formed and added 

 to the tissues on either side, increasing the thickness of the bark 

 on one side and adding to the wood on the other. The delicate 

 cambium is readily torn and forms the Hne along which the bark 

 separates from the wood. 



76. The woody portions of the vascular bundles are arranged 

 side by side around the pithy axis, thus forming the cyHnder 

 alluded to above (paragraph 36). 



77. By experiment it may readily be determined that the 

 fluids absorbed by the roots rise through the stem through the 

 vessels and cell walls of the wood and not through the bark. 

 The same fact is demonstrated by the effect of girdling a tree, 

 which operation does not prevent the rise of the sap nor cause 

 wilting of the leaves. 



78. Other functions of the wood and bark will be noted 

 subsequently. 



Structure and Fimction of the Leaves 



79. In order to fully understand the function of the leaf and 

 the important processes that take place within its tissues it is 

 necessary to study the finer details of its structure by means 

 of the microscope. Thus under moderate magnification a leaf 

 seen in cross section presents the following essential elements of 

 structure : 



80. Both layers of epidermis consist of a single layer of brick- 



