724 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
Other individual anomalies are connected with the leaf-trace bundles in the outer 
part of their course. In most cases the secondary tissues only accompany the out- 
going trace so long as it remains undivided; sometimes, however, both the twin 
bundles of a divided trace possess an external layer of secondary wood and bast.* 
In another case we found one bundle of a pair unthickened, while its twin developed 
an enormous fan of secondary tissue with wood about 40 cells thick (C.N. 1114). A 
more remarkable anomaly of rather rare occurrence consists in the formation of 
cambium all round the leaf-trace bundle instead of on the peripheral side only. An 
instance of this is shown in the first figure of Lyginodendron published.t In another 
stem both the twin bundles of a divided leaf-trace showed the same peculiarity on a 
still larger scale, while another pair approached the same structure. In these cases 
the thickened bundle acquires a concentric structure, and may easily be mistaken for 
the stele of a branch, as actually happened in the first instance referred to. These 
pericyclic anomalies sometimes coexist with abnormal formations in the pith. In one 
case, a little nest of sclerotic cells in the pericycle had served as a centre for a 
concentric development of anomalous xylem and phloém, similar to that around the 
leaf-trace bundles. These varied eccentricities of development are only of interest, 
as showing the same plasticity of structure in these ancient stems as we find in so 
many modern plants, which resemble them in nothing except their mode of secondary 
growth. The anomalous developments in certain Cycadez, among which we should 
most naturally look for parallel cases, bear only a remote resemblance to those which 
we find in Lyginodendron.{ The concentric cortical strands which are formed in 
Cycas appear to be of purely secondary origin. 
Frequent peculiarities, scarcely amounting to an anomaly, consist in the differen- 
tiation of the normal secondary wood into distinct zones, chiefly differing in the 
dimensions of their tracheides. When a narrow zone of small tracheides is formed 
with large elements on both sides of it, an appearance suggestive of annual rings 
may result. These phenomena, however, are far too inconstant for us to draw any 
inference as to a regular periodicity of growth. It is much more probable, that the 
differences between successive zones of wood, which only appear in certain individual 
stems, were due to some accidental interference with the normal course of develop- 
ment. Examples of these peculiarities have been previously figured.§ 
* See Plate 2, photograph 8, also Wituianson, Part IV., Plate 22, fig. 1; Plate 25, figs. 19 and 20. 
+ Wittiamson, Part IV., Plate 22, fig. 1. 
t See pr Bary, ‘Comparative Anatomy,’ Eng. edition, pp. 608-613. 
§ Sec Wituramson, Part IV., Plate 22, fig. 4; Plate 23, fig. 6. 
