158 



LECTURE X. 



here that with this mode of growth, certain differences between the arrangement and 

 form of the secondary cortex and that of the secondary wood necessarily arise. 

 The cambium-ring increases progressively in diameter, and the wood-elements arising 

 from it, and which grow but little in transverse section, need only be" deposited 

 so to speak by apposition on the wood-elements already present; hence the 

 transverse section of the wood comes to present a very evident arrangement of 



its elements in radial rows, and 

 in layers concentric with the 

 periphery. On the outside of 

 the cambium-ring, on the other 

 hand, it by no means suffices 

 that the layers of cortex already 

 existing receive new deposits 

 on the inside ; for the conti- 

 nually progressing increase in 

 circiimferehce of the cambium- 

 ring necessitates a further 

 growth in the peripheral direc- 

 tion of at least a part of the 

 existing'elements of the cortex. 

 In consequence of this, subse- 

 quent alterations of the tissues- 

 elements in the cortex take 

 place, by means of which, under 

 certain circumstances, their ra- 

 dial arrangement is much dis- 

 turbed : the arrangement in 

 concentric layers, on the other 

 hand, is usually more marked. 

 Finally, however, the peripheral 

 growth frequently ceases in the 

 most external (i. e. the oldest) 

 cortical layers ; and then, in 

 consequence of the formation 

 and growth of internal layers, 

 longitudinal cracks arise on the 

 exterior, or other distortions of 

 the outermost, oldest layers of 

 tissue take place. 



When the terms wood and 

 cortex are employed in what follows, the secondary wood and secondary cortex 

 developed from the cambium-ring are always to be understood. We will now 

 consider somewhat more closely the anatomical constitution of these tissues. 



The wood, as well as the cortex, consists of two systems of tissue, the origin of 

 which is already to be seen in the cambium. First, of elements which are elongated 

 longitudinall)', and generally deposited together in the form of bundles or groups of 



Fro. 165. — Diagram of ordinary growth in thickness, with the development of 

 a compact woody mass. A, B, C the same transverse section at different stages in 

 order of age. A before the origin of the interfascicular cambium ; B after its pro- 

 duction; C after the cambium has been active for some time. Everywhere, R 

 primary cortex ; M pith ; p phloem ; x xylem of the vascular bundles ; li t> b three 

 groups of bast-fibres in the phloem. N.B. these are widely separated ih C; Jc 

 fascicular cambium ; ic interfascicular cambium ; /h the wood developed from the 

 fascicular cambium ; i/h the wood developed from the interfascicular cambium ; 

 i/fi the secondary cortical tissue deve'oped from the interfascicular cambium — the 

 diagram is based on drawings of sections of the hypocotyl of Rzcinus cominniiis. 



