ANATOMY 389 
leaf-trace protoxylem, having entered the bundle, runs downwards for one 
internode between but internally to the two lateral strands: at the node 
below it divides into two branches, which curve to the right and the 
left in order to fuse with the neighbouring leaf-traces that enter at this 
node (Fig. So the xylem of the so-called vascular bundle of 
Lquisetum consists of three strands, two of which are lateral and cauline, 
while the median or carinal strand is common to both stem and leaf. The 
fact that only a small portion passes out as a leaf-trace, and not the 
bundle as a whole, constitutes an essential point of difference between 
it and the bundle of a Phanerogam. The general conformation of the 
vascular tissue at the node, according to the 
above description, is shown in the diagram 
(Fig. 213). 
The tracheides in each strand are very 
few, and consequently it is difficult to deter- 
mine the direction of their development. 
However, as regards the leaf-trace and the 
carinal strand it appears clear that they are 
not exarch but endarch, or perhaps slightly Ag f 
mesarch on the adaxial side. The lateral " ene J ' « 
strands, as a whole, are differentiated later 
than the carinal strand, but they do not 
seem to be a continuation of its centrifugal 
development. On the contrary, in 2. gzgan- 
teum, where as many as ten to fifteen elements 
2iI2\)s 
Fic. 213. 
Diagram constructed by Mr. Gwynne- 
Vaughan to represent the conformation 
of the vascular tissue at a node of Eguz- 
setum. Above and below the node the 
groups of three strands, the median leaf- 
are present in each lateral strand, the smallest 
of them are invariably at the outer extremity, 
and they gradually increase in size inwards. 
Longitudinal sections show that the largest 
trace and the lateral cauline strands, are 
shown. Inthe centre the entry of a leaf- 
trace is shown, and its passage inwards 
to take its place nearest to the pith, 
while the cauline strands are lateral, and 
form the external parts of the composite 
vascular bundle. 
tracheides are coarsely reticulate, with large 
pits and very broad bands of thickening between them: in the smaller 
elements the reticulation becomes finer and more regular, and in the 
smallest it closely resembles true spiral thickening. To state definitely 
whether the lateral strands are exarch or not was not possible in this 
species, because no incompletely differentiated portions of the stem were 
available: so the question must remain at present undecided, although 
the mature structure certainly gives a strong impression of centripetal 
development. 
It is suggested by Gwynne-Vaughan that the lateral xylem-strands in the 
vascular bundles of the existing species of Lgzsetum may perhaps be 
taken to represent the last remnants of a primitive central mass: this 
would be in entire agreement with their apparently centripetal develop- 
ment, and in particular with their cauline course. The probability of 
this suggestion can best be gauged by comparison with the fossil Equisetales, 
and with other Pteridophytes. For Ca/amites the case has been succinctly 
