158 



LECTURE X. 



here that with this mode of growih, 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 

 circumference 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 tissue- 

 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 

 th the development of conscquencc of the formation 



■n at different stages ni '■ 



and grow^th of internal lawyers, 

 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 



FIG. 165.— Diagram of ordinary growth in thickni 

 a compact woody mass. ^, B, C the same transverse 

 order of age. A before the origin of the interfascicul 

 duction; C .ifter the c.imbiimi has been active for 

 primary cnrt. \ : 1/ lili : - |]liloeni ; x xylein of tlie \ 

 groups of t ' : phloem. X.B these ar. 



fascicular c,i . ;.- , ,;/ >rl,iscicular cambium ; fh t 



fascicular cdiui.uim ; i/'i ilie wood developed from 



ir cambium ; B after its pro- 

 ome time. Everywhere, R 

 iscular bundles \ b b b three 

 widely separated in C\ fc 

 le wood developed from the 

 lie interfascicular cambium ; 



i/p the seconder; 

 diagram is based 



tissue deve'oped from the interfascicular cambium— the 

 1 of the hypocotyl of Ricinus cotnj/nniis. 



