in the ‘ Seedlings' of Certain Leptosporangiate Ferns. 403 
word, the external phloem becoming continuous with the internal at each 
exit of a trace, and the endodermis merely stretching between the horns of 
the temporarily broken rod. It will be noticed that there is no question 
of a primitive xylem rod with a central strand of 4 extra-stelar ’ parenchyma ; 
still less is there any evidence of an internal endodermis. We merely have 
an amphiphloic vascular rod to which are attached, in the simplest manner, 
the vascular bundles which supply the leaves. Subsequent processes are 
merely an elaboration of this primitive strand, the resulting structure 
no doubt representing the most satisfactory compromise in meeting the 
requirements of several demands, prominent among which is the necessity 
for an efficient mechanical distribution of the tissues concerned. The steps 
by which the mature condition of the vascular tissue is reached are very 
simple. 
At about the fourth or fifth leaf-trace, the gap is no longer very small, 
but of such a size that ground-tissue is differentiated between the horns 
of the gap, the ground-parenchyma everywhere being separated from the 
vascular tissue by the endodermis. Before the gap is closed, the vascular 
arc breaks at a point opposite it, and, from each of the two horns, a strand 
is nipped off, the two forming the first double leaf-trace. Soon afterwards, 
the first gap closes and is followed by a re-splitting for the next trace. In 
older plants, the overlapping of the gaps is the most striking feature, and 
a simple type of dictyostely is attained without the previous appearance of 
Jeffrey’s siphonostele, consisting of an amphiphloic vascular ring with 
an internal and external endodermis. 
Although the vascular system of Doodia aspera is undoubtedly the 
simplest type examined, the writer is of opinion that it does not represent 
the true phylogenetic development of the vascular system of the Poly- 
podiaceae. It is best considered as a simplification of that other type 
of vascular elaboration which was found to be so general among the plants 
investigated, viz., that in which an early appearance of a ground-tissue 
pocket was a characteristic feature. It will be remembered that the appear- 
ance of the first ground-tissue pocket usually occurs at that region of the 
plant at which the leaf-traces show a transition from the single to the double 
character. This transition is not met with in Doodia aspera , and, further, 
the first double leaf-trace appears relatively early in this plant, supplying as 
it does the fifth leaf, whereas in the majority of other forms examined it did 
not appear until the level of the sixth or seventh leaf. That is to say, the 
stage at which, from the study of other forms, we should expect to meet 
with a ground-tissue pocket is omitted in this plant. To put the matter 
briefly, the type of vascular elaboration met with in Doodia aspera is 
probably best regarded as a 4 short cut ’ to the adult dictyostelic arrange- 
ment. In the type exhibiting a ground-tissue pocket, the protostele, as 
usual, soon gives way to a ring of xylem surrounding and surrounded 
