2^2 Barratt. — A Contribution to our Knowledge of the 
Whatever factors have been concerned in effecting the gradual lack of 
differentiation of wood recognizable as such, the actual areas of the xylem- 
in which the replacement of tracheides by parenchyma has taken place have 
had no necessary relation to the insertion of leaves and branches. The 
present distribution of the xylem, and hence of the parenchymatous tracts, 
is such as to best preserve the efficiency of the wood in its double function 
of water-conduction and mechanical support. In the internodes of the 
vegetative axis, where the reduction of xylem has been carried very far, it 
has no doubt made possible the great elongation which is such a marked 
characteristic of the genus. The rigid tube of short tracheides at the nodes 
is an important factor in maintaining the mechanical efficiency of the stem. 
In the cones where the nodes are absent the greater development of meta- 
xylem and its more equal distribution is adapted to provide for the support 
of the numerous sporangiophores. 
In conclusion, it may be stated that in the writer’s opinion the size and 
distribution of the tracts of parenchyma have no morphological value in the 
discussion of questions of phylogeny. The same may be said of that much- 
debated tissue the endodermis. In Equisetum not only does it appear in 
various relations to the vascular tissues in different parts of the same plant, 
but its arrangement also varies in the corresponding organs of different 
species. Kashyap (23) has shown in E. debile that small groups of paren- 
chymatous cells without any vascular elements may be surrounded by 
separate endodermes. The anomalous tuber described in the present paper 
may also be instanced. 
It may be recalled in this connexion that similar cases of unusual and 
independent occurrence of an endodermal layer have been recorded in other 
plants, e. g. in the roots of Ruscus (Lewis, 24). 
Whatever factors may have determined its distribution, they are 
probably physiological ones, which, however, can scarcely be satisfactorily 
elucidated in the present state of our knowledge. 
Summary of Results. 
i» The sporeling of EquiseUnn arvense shows a protostelic condition at 
its base which opens to a siphonostele at the level of attachment of the 
vascular supply of the secondary axis. The protostelic condition is again 
resumed for a short distance below the level of the attachment of the first 
whorl of leaves. 
2. The basal regions of the succeeding axes of the young plant possess 
a compact closed siphonostele composed of short reticulate tracheides. 
There is thus formed a sympodial vascular tube in which five or more axes 
may be concerned. 
3. The secondary axis arises endogenously from the primary axis 
below the level of the first leaf-whorl. 
