''Ill-] THE BEAN-PLANT. 71 



)f the cambium layer, to the thickness of the wood, and on 

 ,he outer side of it, to the thickness of the bark ; and the 

 ixis of the plant continually increases in diameter, so long as , 

 his process goes on. Plants in which this constant addition 

 .0 the outer face of the wood and the inner face of the bark 

 akes place, are termed exogens. 



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

 :ambium layer is continuous with the cells which retain 

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

 ;hickest at the junction of the stem and root, and diminishes 

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

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

 ;one. And it is the special peculiarity of an exogen to possess 

 ;his doubly conical layer of constantly dividing cells, the upper 

 ;nd of which is free, at the growing point of the terminal 

 jud of the stem, while its lower end is covered by the root- 

 !ap of the ultimate termination of the principal root. 



The most characteristic tissues of the wood are dotted 

 lucts and spiral vessels, the spiral vessels being particularly 

 ibundant close to the pith. The bark contains elongated 

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

 IS are found in the Fern. 



Stomates are absent in the epidermis of the root: they 

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

 p-een parts of the stem and its appendages, but, as in the 

 ^'em, they are most abundant in the epidermis of the under 

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

 ntercellular passages, which are widest in the leaves, but 

 ixtend 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 

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

 )ut the extreme terms of one series of modifications. The 



