264 THE ANATOMY OF STEMS. 



vergent layers. The individual cells, which are nar- 

 row and horizontal in their length, extend in series 

 from the centre to the circumference of the wood ; 

 and consequently form nearly right angles with the 

 tubes of the concentric layers. They communicate 

 with each other by pores ; so that fluids may readily 

 pass through the whole series, and of course trans- 

 versely through the wood ; and Mirbel remarks that, 

 " in many coniferous trees the divergent rays are not 

 cellular, but consist of horizontal tubes, which extend 

 from the pith to the bark." Whether they are cellu- 

 lar or tubular, the layers, or masses, are flat, or in 

 plates, with the edges placed vertically and thicker 

 in the centre than either above or below, appearing 

 therefore of a lozenge shape when vertically divided ; 

 whilst in their transverse section they display a slight 

 inclination to the wedge form. They are much more 

 delicate in their structure than the concentric layers ; 

 and readily dissolve, like the common cellular texture, 

 so that when a thin tangental slice of wood is macerat- 

 ed in water, the divergent layers are decomposed, 

 and leave the meshes of the concentric layers empty, 

 displaying the appearance of a net-work or lace simi- 

 lar to that formed by the macerated liber. From the 

 cellular texture of the divergent layers, they are re- 

 garded by some authors as processes of the pith; and 

 hence have been named medullary rays ; but many 

 of them cannot be traced to the pith, although the 

 more conspicuous of them traverse the whole of the 

 wood, from the pith to the bark. 



Wood, while in the state of alburnum, is endowed 

 with nearly as much irritability as the liber, and per- 

 forms functions of great importance in the vegetable 

 system ; but when hardened, these functions cease, 

 and in time it loses even its vitality ; not unfrequently 

 decaying in the centre of the trunk of trees ; which. 



