THE CAMBIUM 



64<J 



cambium. Ordinarily the cambium takes the form of a hollow cylinder, 

 which in transverse section, of course, appears as a complete ring. 



II. SECONDARY GROWTH IN THICKNESS IN 

 GYMNOSPEEMS AND DICOTYLEDONS. 



A. THE CAMBIUM. 349 



In the stems of Gymnosperms and Dicotyledons, the primary 

 vascular bundles are intersected by the cambial cylinder at the junc- 

 tion of hadrome and leptome, the former being intra-, and the latter 

 extra-cambial in position ; the development of secondary tissue thus 

 continually pushes the primary components of the bundles further 

 apart in the radial direction. 



The mode of origin of the cambium varies considerably in different 

 axial organs. Frecpiently it stands in close relation to the primary 



ABC 



mm L #?pe 



Fig. 26S. 



Successive stages in the differentiation of the primary procambial cylinder in the 

 stem of Salvia Horminum (T.S.) See text. 



differentiation that takes place in the apical region, while in other cases 

 there is no connection between the two processes. 



In a number of stems the cambium arises from a primary pro- 

 cambial cylinder. A couple of examples will render this statement 

 clearer. A young internode of Salvia Horminum contains eight 

 vascular bundles (leaf-trace bundles), a pair being situated in each 

 angle of the four-sided stem. The two strands of each pair soon fuse 

 to form a single compound bundle. At an early stage of develop- 

 ment, four strips of procambial meristematic tissue arise between the 

 compound bundles ; these gradually extend parallel to the flat sides of 

 the stem, and become connected at either end with the fascicular cam- 

 bium. The complete primary procambial cylinder formed in this way, 

 however, by no means represents the actual cambium. The interfas- 

 cicular strips of procambial tissue comprise three or four layers of cells, 

 which are arranged quite irregularly (Fig. 268 a), and not in the 



