404 Chandler. — On the Arrangement of the Vascular Strands 
by phloem ; the early leaf-traces are quite simple, and then suddenly in the 
central phloem we have the appearance, in surface view, of one or two 
endodermal cells, followed immediately by definite ground-tissue cells 
surrounded by a ring of elements possessing all the well-known characters 
of an endodermis. These phenomena may possibly be interpreted as 
meeting the demands of the plant for a more peripheral arrangement of its 
vascular tissues than has obtained hitherto, the demand being met by the 
differentiation, from the meristem, of a central cone of ground-tissue con- 
tinuous with the general mass of ground-tissue, and with its apex pointing 
downwards. Certainly a most striking fact in connexion with this ground- 
tissue pocket is that the fundamental-parenchyma is at all points shut 
off from the vascular tissue by a continuous sheath of endodermal cells. It 
is quite unnecessary to describe the further changes resulting in the perfec- 
tion of the mature dictyostelic structure, since they have been given in full 
elsewhere. The interest of these plants lies in the fact that, at a level 
immediately succeeding the appearance of the internal endodermis, we have 
a very transitory condition of affairs exactly resembling, in an isolated 
section, the structure of the primitive siphonostele demanded by Jeffrey’s 
hypothesis (Fig. 201). 
In the light of what has been previously said as to the origin of this 
ground-tissue pocket, its essential structure, and its relationship to the 
general mass of ground-tissue, the writer believes that the assignment of 
a morphological significance of fundamental importance to such an arrange- 
ment of tissues is, perhaps, unjustifiable. The reason for the appearance 
of the internal ground-tissue appears to be fairly obvious, if, with Jeffrey, 
we admit the important part which mechanical considerations must play in 
the elaboration of the vascular system. The plant, at the time when the 
pocket is differentiated, apparently requires a more peripheral arrangement 
of its vascular tissues, and the dilatation of the existing conducting-tissue 
is probably the least expensive and most efficient means of meeting the 
requirements of the case. Before the necessity for such dilatation arises, 
the vascular strand is amphiphloic in structure, and since the separation 
of vascular from non-vascular tissue is so very generally effected by the 
presence of an endodermal sheath, the differentiation of internal ground- 
tissue naturally results in the appearance of an amphiphloic and amphi- 
endodermic vascular ring to which it is unnecessary, perhaps, to ascribe any 
profound significance. 
The position, therefore, which the writer is inclined to take up with 
regard to the evolution of the dictyostelic vascular system is that the latter 
is derived from a certain simple primitive type, as a result of meeting the 
demands of several requirements, the chief and most obvious of which are, 
(1) the efficient distribution of the vascular tissue ; (2) the increase of the 
conducting system in the young plant. The primitive type is not regarded 
