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Vascular System of the Genus Equisetum. 
A considerable amount of material was looked over in order to find 
other specimens with a similar structure, but without success. It seems 
clear, however, from this one case that the xylem in the bundle of the 
normal tuber consists of a relatively large mass of tracheides comparable to 
the lateral strands of metaxylem found in the typical vegetative internodes, 
while the protoxylem associated with the carinal canals of the latter is 
usually indistinguishable in the mature tubers. It is not suggested that 
this is not present at all, but if formed is there in very small amount, and 
owing to the tremendous development of parenchyma and consequent 
radial extension the small canal formed by its disruption becomes obliterated 
and the remains of the elements are no longer visible. It has already been 
mentioned that the protoxylem elements of the abnormal tuber are 
extremely small, and would probably have been overlooked had it not been 
for their separation from the main elements of the bundle and their inclusion 
within a separate endodermis. 
The full significance of this structure will be evident when compared 
with the cone and vegetative axis. 
Secondary Thickening. 
Since the appearance of Cormack’s (15) observations on the xylem of 
E. maximum , the view has been very generally held that a limited amount 
of secondary growth takes place in the xylem of the nodes of certain 
species of Equisetum. Cormack (15) supported his conclusions by a com- 
parison between the number of elements present in the nodal groups of 
xylem at the apex and in the same relative position several internodes 
lower in the stem. He shows that in the successive nodes the number of 
elements of lignified tissue increases, whereas the number of elements of 
the internodes remains more or less constant. This observation, which is 
readily confirmed, does not necessarily prove the point, for the increase in 
number of elements as seen in transverse section may be due to other 
causes than multiplication by cambial cell division. Longitudinal sections 
were examined in order to determine whether this ‘ increase ’ at any 
particular level had not been produced by the elongation of the elements in 
question accompanied by displacement due to sliding growth. 
To determine whether this has actually happened, it is necessary to 
measure the lengths of the elements in the various regions and to examine 
critically the relation existing between the cells of the nodes and the inter- 
nodes. In E. arvense, some dormant underground buds were fixed and 
sectioned in a longitudinal direction. These buds were particularly 
favourable for such an investigation because the numerous nodes in each 
bud showed every stage in the development of the xylem. 
Similar buds of E. maximum were also examined and the same con- 
ditions were found. The drawings and measurements were, however, made 
