A Note on Vascular Tissue. 
195 
strand a parenchymatous pith arises. On the inner face of the 
tracheidal tube phloem may appear and an internal endodermis then 
cuts this off from the central parenchyma; this parenchyma is still 
regarded as part of the central cylinder. By the development of a 
cylinder of vascular tissue, incomplete in certain places, the dictyo- 
stelic condition supervenes, the external and internal endodermis 
being continuous through the gaps. By the overlapping of these 
gaps a very complicated open network—such as is found in Aspidium 
—results. I n the ferns, then, gaps are intimately associated with the 
development of leaf-traces. Through the gaps the internal— 
according to this view “stelar”—and the external—“ extrastelar ” 
parenchyma becomes continuous. Yet, although the parenchyma 
of external and internal origin may be precisely similar and abso¬ 
lutely continuous, it is held that a very essential difference exists 
between them, although the boundary cannot be determined. Hence 
a purely abstract consideration is introduced. The importance of 
the endodermis as a morphological boundary between intra- and 
extra-stelar tissues is abandoned, since, if it were maintained, the 
only possible conclusion would be that in the dictyostelic form the 
intra-cylindrical and extra-cylindrical parenchyma are of the same 
nature— i.e., both “extra-stelar,” the xylem and phloem alone repre¬ 
senting the original protostele. It is indeed quite necessary to this 
theory that the endodermis as a morphological layer be abandoned. 
Hence it is rather curious to find Mr. Boodle distinctly stating (4) 
that “ although the presence of an endodermis is probably no guide, 
its local absence may have some morphological significance.” 
This statement refers to a condition in certain palm-roots where 
the endodermis is locally absent between xylem and parenchyma. 
It certainly seems to follow from this statement that Mr. Boodle 
holds the absence of an endodermis may prove that morphological 
continuity exists even between vascular tissue and parenchyma. It 
may justly be urged that much more would it tend to indicate 
such a morphological continuity in a continuosly developed tissue 
wherein no difference of any sort can be seen. Still Mr. Boodle 
refuses to accept this view, at the same advocating the other. 
Apparently Mr. Boodle holds that development gives no clue to 
morphological relationships of tissues—at all events that apical de¬ 
velopment is valueless. Now it would seem that many plants, perhaps 
all,—which in the mature state possess a dictyostelic arrangement, 
commence life with a protostele. The successive stages of develop¬ 
ment culminating in the dictyostele can be readily followed. Here, 
we should have thought, is a very valuable piece of evidence to the 
