ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 747 
2. Course of the Vascular Bundles. 
The main facts as to the course of the vascular bundles in Heterangium Grievii 
have always been understood, since the first description of the plant was published in 
1872. A series of eight transverse sections from a piece of stem nearly two inches 
long, figured in Part IV., Plate 30, affords conclusive proof that the vascular bundles, 
which are found in the cortex, pass gradually outwards and enter the bases of the 
leaves, each of which receives a single bundle. At the same time the lower ends of 
these bundles were traced downwards to their point of union with the central 
cylinder.* 
It has only been necessary for us to consider two points :— 
1. What light does the course of the bundles throw on the phyllotaxis of the 
plant ? 
- 2, To what extent can the leaf-trace bundles be recognized, as distinct strands, at 
the periphery of the primary wood ? 
Some of the specimens are sufficiently complete and well-preserved to enable us to 
determine the succession of the leaf-trace bundles in the same way as was done for 
Lyginodendron (p. 708). 
In some of the larger stems as many as nine or even ten leaf-traces are seen in the 
transverse section, counting all those which have once begun to separate from the 
central cylinder. Their arrangement points distinctly to a 2 divergence.t In one 
case, for example, the 9th bundle (counting from within outwards) lies directly 
outside the 1st, while the 10th corresponds in like manner to the 2nd. Here the 
phyllotaxis must evidently have been 3. It is possible that a higher divergence may 
have sometimes occurred. In the smaller stems a } arrangement has been observed 
(C.N. 1298). 
As regards the second question, the careful observation of good transverse sections 
shows clearly that the arrangement of the tracheze in the primary wood is not an 
irregular one, but that a number of definite bundles can be distinguished at the 
periphery of the stele. These peripheral strands have essentially the same structure 
as the xylem of the leaf-trace bundles, with which they are in continuity.{ 
We have now determined that each of the peripheral groups corresponds very 
exactly in structure with one of the perimedullary strands of xylem of Lyginodendron. 
In Heterangiwm the number of these strands at the periphery of the cylinder is very 
large—(from 16 to 20)—greatly in excess of the number of orthostichies which 
appear to have existed. This is a point of difference from Lyginodendron, in which, 
* Wittiamson, “ Organization,” Part IV., p. 401; Plate 28, fig. 30, m'’, &e. 
+ The position of the two leaves of which the bases are shown in the series figured in WiLLIamson, 
“ Organization,” Part [V., Plate 30, also indicates a 3 phyllotaxis. 
t See figs. 21, 24, and 26; also the transverse sections figured in “ Organization,” Part IV. 
5 D 2 
