456 SECONDARY CHANGES. 



and then extends, in succession, to those which are less thick. Tn addition to the 

 example represented in Fig. 194, the process described occurs in the Menispermese, 

 Casuarinse, and Begonias investigated, in Berberis and Mahonia^ in Aristolochia Sipho 

 and its allies, Atragene, and the woody Piperacese. Cucurbita is also to be men- 

 tioned here, in so far as the two concentric rings of bundles in its stem (p. 248) 

 behave, as regards the relation in question, as a single ring curving alternately inwards 

 and outwards. 



In Cucurbita, as well as in Aristolochia Sipho and Atragene, the behaviour 

 described is most clear, because it goes on especially slowly. In the former plant 

 particularly the growth in thickness by means of meristematic new formation first 

 goes on in the vascular bundles alone, the cells of the parenchymatous medullary 

 rays following it for a long time only by means of radial extension; only at a 

 relatively late period does tangential division begin in the latter, in the order 

 described. 



2. No doubt in the majority of cases, the first appearance of the cambium is 

 immediately preceded by the formation of cauline intermediate bundles, and this 

 happens as follows : — 



(a) In the broad medullary ray between two leaf-trace bundles, distinct collateral 

 vascular bundles appear, which are again separated from the leaf-traces by distinct 

 medullary rays. The formation of the cambium then starts from the two kinds of 

 bundles, in the same way as in (i). This process takes place in a very evident 

 manner in the internodes of Clematis Vitalba. When the completion of the six leaf- 

 trace bundles (p. 244) has begun, an intermediate bundle appears between each two 

 of them in the usual manner (p. 389). The twelve bimdles of the ring which are then 

 present are separated from one another by the same number of radial bands, each 

 about ten cells broad. The cells of the latter now assume parenchymatous properties, 

 while the twelve vascular bundles increase and develope their elements in the radial 

 direction; finally, the completion of the cambial ring takes place by means of 

 tangential division of an interfascicular layer of cells, starting from the edge of the 

 six leaf-trace bundles. Each intermediate bundle runs longitudinally through the 

 entire internode, and is only attached by its ends to the trace-bundles, in the nodes. 

 A further case, belonging to this category, and characterised by. the very early 

 appearance of the intermediate bundles, is represented by the Rhipsalidese with 

 winged corners to the stem, spoken of at p. 261. 



How far the same process takes place elsewhere in internodes is not certainly 

 known at present ; the ' complementary bundles ' of Ephedra campylopoda, noticed 

 above at p. 247, may belong to the same category. It at any rate occurs very 

 generally in the hodes, even among forms belonging to category i, in order to 

 form the oblique or reticulate connections of the trac? bundles, which here appear 

 in all cases at an early period. 



(p) Between each two leaf-trace bundles collateral intermediate small bundles 

 of normal orientation appear, consisting only of one or a few radial rows of 

 elements, and separated both from the leaf-trace bundles and from one another by 

 equally narrow, non-equivalent radial bands (parenchyma). Their longitudinal 



Sanio, Botan. Zeitg. 1863, p. 373. 



