464 THE BEAN PLANT. [CHAP. 



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 bast ; and the axis of the plant 

 continually increases in diameter, so long as this process 

 goes on. This constant addition to the outer face of the 

 wood and the inner face of the bast is characteristic of the 

 Dicotyledons and Gymnosperms, to which two groups all our 

 forest trees belong. In the Bean this process of secondary 

 thickening only goes on to a comparatively small extent. 



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

 cambium layer is continuous with the cells of the apical 

 meristem 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 layer of cambium and 

 meristem may be said to have the form of a double cone. 

 And it is the special peculiarity of the groups of plants 

 above-mentioned 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 pitted and 

 spiral vessels, the spiral vessels being particularly abundant 

 close to the pith. They are the first elements of the xylem 

 to be formed. The outer part of the bast consists of elon- 

 gated bast fibres, while the inner, or soft bast, contains 

 sieve-tubes, the transverse walls of which are perforated. 



Stomata 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 abundanfin the epidermis of the under 

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



