viii.] THE BEAN-PLANT. 73 



their power of multiplication, and divide by septa parallel 

 with the length of the stem, or root, as well as transverse to 

 it. Thus new cells are continually being added, on the 

 inner side of the cambium layer, to the thickness of the 

 wood, and on the outer side of it, to the thickness of the 

 bark; and the axis of the plant continually increases in 

 diameter, so long as this process goes on. Plants in which 

 this constant addition to the outer face of the wood and the 

 inner face of the bark takes place, are termed exogens. 



At the apex of the stem, and at that of the root, the 

 cambium layer is continuous with the cells, which retain 

 the capacity of dividing in these localities. As the plant is 

 thickest at the junction of the stem and root, and diminishes 

 thence to the free ends, or apices, of these two structures, 

 the cambium layer may be said to have the form of a double 

 cone. And it is the special peculiarity of an exogen to 

 possess this doubly conical layer of constantly dividing 

 cells, the upper end of which is free, at the growing point 

 of the terminal bud of the stem, while its lower end is 

 covered by the root-cap of the ultimate termination of the 

 principal root. 



The most characteristic tissues of the wood are dotted 

 ducts and spiral vessels, the spiral vessels being particularly 

 abundant close to the pith. The bark contains elongated 

 liber or bast cells; but there are no scalariform vessels such 

 as are found in the Fern. 



Stomates are absent in the epidermis of the root: they 

 are to be found, here and there, in the epidermis of all the 

 green parts of the stem and its appendages, but, as in the 

 Fern, they are most abundant in the epidermis of the under 

 side of the leaves. As in the Fern, they communicate with 

 intercellular passages, which are widest in the leaves, but 

 extend thence throughout the whole plant. 



