696 TRANSACTIONS OF SECTION K. 
is a closed cylinder, without gaps at the insertion of the leaf-traces. In such 
plants the vascular cylinder is as well defined as in all roots, and can be described 
in the same terms. But the case is quite different in the stems of Phanerogams, 
where to all appearance the primary vascular cylinder is a system built up of 
leaf-traces, embedded in a parenchymatous matrix. And the early anatomists 
were faced at once by this problem in its crudest form. Beginning with the 
anatomy of Phanerogams they first became acquainted with the primary structure 
of the Dicotyledonous stem. That of the root was not clearly understood until 
many years later; perhaps because anatomists attempted to interpret it by 
reference to the skeleton of the stem, and in the same terms. But there is 
nothing in the vascular anatomy of the root to correspond with the leaf-trace, 
and the leaf-trace is the vascular unit of stem-structure in all Phanerogams. 
Here, as elsewhere, confusion of nomenclature went hand in hand with confusion 
of thought, and it is difficult to say which was cause and which effect. 
Even when the facts of root-structure were accurately known, the conception 
of the leaf-trace bundle as the structural unit continued to be a stumbling- 
block. In 1877 De Bary published his monumental work on Plant Anatomy, 
and, though it still keeps its place as the great book of reference on that 
subject, his descriptions of root anatomy appear to the modern botanist to be 
written in a dead language. When he calls the vascular axis of the root a 
‘radial bundle’ it is quite clear that he regards this as a purely formal term, 
not implying any true homology between the leaf-trace bundle of the stem and 
-the axial core of the root. He does not, indeed, consider a bundle as a unit: 
he defines it as a compound structure ‘formed of tracheids and sieve-tubes 
definitely grouped.’? But the word ‘bundle’ was already impressed with 
another superscription. However defined originally, it had connoted the unit 
of stem-structure to a generation of botanists. With that connotation, De Bary’s 
use of the term is in hopeless conflict. Moreover, the conception underlying 
that use was already out of date in 1877. Modern anatomy dates from 1871, 
when Professor Van Tieghem published the first of his great series of memoirs 
on the subject. In these the axial core of the root was treated as equivalent 
to the whole system of leaf-trace bundles in the stem, though the word ‘stele’ 
was not yet invented. This conception gained ground from the first; it was 
popularised by the happy choice of a name in 1886. From that date the stelar 
hypothesis has replaced all other schemes of vascular anatomy. The advance 
then made on all previous generalisations has been shown by the new impulse 
given to research, and the comparative simplicity introduced into text-book 
anatomy. 
We cannot claim equal simplicity, I fear, for the technical language of 
research in this subject, and this alone should inspire caution, for obscurity of 
language rarely persists where there is no corresponding obscurity of thought. 
No one now doubts that the central cylinder of the root in Phanerogams is 
far more closely comparable to the leaf-trace cylinder of the stem than to any 
one of the traces within it. Yet when the comparison becomes detailed 
difficulties are constantly arising. Where, for example, there is a medulla in 
the root it certainly forms part of the stele, which is a solid cylinder sharply 
defined by the specialised endodermis surrounding it. But the leaf-traces in 
the young stem surround a massive cylinder of parenchyma, precisely resembling 
the parenchyma of the cortex, with which it is in apparent connection through 
the gaps between the leaf-traces. Even the secondary formations do not com- 
pletely divide one system from the other. When a specialised endodermis is 
present it is not so clearly defined as in the root : in many cases it is not present— 
in other words, there is no cell-layer outside the leaf-trace cylinder which is 
differentiated in any way from the surrounding tissues. In a few instances— 
most baffling of all—an endodermis surrounds each lJeaf-trace. 
The stele in the stem of Phanerogams is not of necessity a morphological 
fiction because in many stems its precise limits cannot be determined. If, 
indeed, the word be used as a descriptive term its value is seriously impaired 
by every instance in which it fails to describe stem-structure with precision. 
. Sia ibe Anatomy of Phanerogams and Ferns. 1st Eng. ed., 1884, 
p. 400. 24 ts : ; 
