THE CAMBIUM 651 



yet be a heterogeneous structure when considered from a morphological 

 point of view. 



The last-described mode of cambial development shows some 

 variation of detail in different cases. Typically (Menispermaceae, 

 Castcarina, Aristolochia Svpho, Eieinm communis [hypocotyl], Begonia 

 [Fig. 269], Oiicurbita, etc.), secondary meristematic activity is at once 

 directed to the formation of the interfascicular cambium ; in a few cases, 

 however (e.g. in Clematis Vitalba), the completion of the cambial 

 cylinder is preceded by the differentiation of secondary cauline bundles, 

 which appear in the medullary rays between the leaf- trace strands. 



In the roots of Gymnosperms and Dicotyledons, the cambium 

 originates in the central cylinder. Tangential division starts on the 



Fir;. 269. 



T.S. through the stem of Begonia Juchsioides, passing through a vascular bundle. 

 The fascicular cambium is continuous with interfascicular cambial strips on either 

 side, x 150. 



inner side of the leptome-strands and thence extends laterally through 

 the conducting parenchyma as far as the outer ends of the hadrome 

 plates. The linking up of the separate strips of secondary meristem 

 into a complete cambial cylinder is effected, by the cells of the pericycle 

 (pericambium) outside each xylem plate becoming meristematic (Fig. 

 270). Here again the leptome is extra-cambial, and the hadrome intra- 

 cambial. In other words, the orientation of the primary components of 

 the vascular bundle with regard to the cambium is identical in the 

 root and in the stem, and the subsequent course of secondary growth 

 may also be the same in the two organs. On account of its mode of 

 origin, the cambial cylinder of a root is, to begin with, curved inwards 

 opposite each leptome-strand ; as the growth in thickness proceeds, 

 however, the primary leptome is rapidly pushed outwards and the 

 cambium acquires an approximately circular outline. 



