ANOMALOUS THICKENING IN DICOTYLEDONS AND GFMNOSPERMS. 609 



the one to the right, the other to the left, and curving downwards through the cortex, 

 they finally enter the bundle-ring, and there descend further in a radially perpen- 

 dicular manner. This last part of their course, and their final lower insertion, has 

 not been thoroughly investigated. Th^ length of the curve, projected in the 

 horizontal plane, and regarded as circular, which each bundle of the trace describes 

 through the cortex before its entry into the ring of bundles, is not exactly defined, 

 and appears not to be always the same, but has been estimated in the case of 

 Cycas revoluta at least at 145° to 150°, but does not reach 180°, that is the coalescence 

 of the two bundles. At the beginning of its course, as followed from the leaf- 

 insertion downwards and inwards, each bundle passes for a long distance imme- 

 diately below the surfaces of insertion of the leaves, that is just within the surface of 

 the cortex of the stem, diverging but slightly from it, and running not exactly 

 horizontally, but descending only slightly. In the last portion of its course, which is 

 approximately equal in height to that of one leaf-in&erlion, it runs more steeply 



Fig 



'19. 



Fig. 240. 



Fig. 239. — Cycas revoluta. Course of the bundles of the leaf-trace, in a thick transverse section closely below the 

 punctum vegetationis of a lateral shoot, made transparent with potash, and seen from its acroscopic surface ; slightly 

 magnified. The bundles, which run in different, but closely superposed planes, are drawn together m the plane of the 

 paper, so that the point of exit is kept most dark, and each bundle is drawn from this point to that at which it curves 

 steeply downwards ; the bundles of the eight successive yoimgest leaves are successively numbered, i, those of the 

 youngest, highest, &c.; those of the oldest, 9, are not numbered. Between 4, 5, 7, and 8, radial connections are 

 beginning. 



FIG- 240 — Cycas revoluta. Transverse section with six bases of leaves, near to the apex of a small plant raised 

 from a lateral bud ; natural size. Parenchyma of pith, cortex, and leaves left white. The vascular bundles laid bare by 

 the section are represented as lines, those cut transversely as dots. The inner circle is the still ven narrow young ring 

 of bundle? surrounding the pith ; it consists of the lower portions of bundles of the trace descending from above Out- 

 side it are the delicate beginnings of the radial connections. 



inwards and downwards, till it enters the ring of bundles. The condition described is 

 accurately represented only while young. A thick transverse section through the 

 shortly-conical end of the leafy stem, if made transparent, shows the leaf-traces of 

 the youngest, uppermost leaves in the young cortex as simple symmetrical pairs of 

 curves, which increase in width the further they are from the apex (Fig. 239). 

 Closely below the conically diminished end, the stem and the ring of bundles assume 

 a more or less cylindrical form ; when, or even before the curves come, by means 

 of the progressive longitudinal growth of the apex, to lie in the cylindrical region, 

 the original arrangement is modified by the appearance of connecting branches. 

 These are in the first place so directed as to follow the course of the curves them- 



R r 



