VIII.] THE BEAN-PLANT. 71 



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. 



The difference between a flowering plant, such as the 

 Bean, and a flowerless plant, such as the Fern, at first sight 

 appears very striking, but it has been proved that the two are 

 but the extreme terms of one series of modifications. The 



