346 The British Association at Birmingham. 
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 apparently the primary vascular cylinder is a system built 
up of leaf-traces embedded in a parenchymatous matrix. The early 
anatomists were faced at once by this problem in its crudest form, 
for they began with the primary structure of the dicotyledonous 
stem, and that of the root was not clearly understood until many 
years later ; since they attempted to interpret it by reference to the 
skeleton of the stem and in the same terms; though there is nothing 
in the anatomy of the root to correspond with the leaf-trace, and 
the leaf-trace is the vascular unit of stem structure in all Phanero¬ 
gams. 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. Modern anatomy dates from 
1871, when Van Tieghem published the first of his series of memoirs 
in which the axial core of the root was treated as equivalent to the 
whole system of leaf-traces in the stem ; a conception which gained 
ground from the first, and was popularised by the happy choice of 
the term “ stele ” in 1886. From that time 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. It is generally accepted 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 arise. For instance, where there is a pith in the root it 
certainly forms part of the stele, which is a solid cylinder sharply 
defined by the specialised endodermis around it; but the leaf-traces 
in the young stem surround a massive cylinder of parenchyma 
exactly resembling the parenchyma of the cortex with which it is in 
apparent connection through the gaps between the leaf-traces. 
Even the secondary formations in the stem do not completely 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, and in a few instances there is an endodermis around 
each leaf-trace. However, the stele in the stem of Phanerogams is 
not necessarily a morphological fiction because in many stems its 
precise limits cannot be determined, for morphology is not merely 
descriptive. If we suppose that the stem stele in remote ancestors 
of the Phanerogams was as well defined as that of the root, and 
clearly comparable to it, we may attach a real morphological 
meaning to the term when applied to modern Phanerogams, pro¬ 
vided we can show cause to believe that what we call the stele in 
their stems represents the ancestral stele. Its tissues will then 
have a history distinct from those of the cortex, though not clearly 
separated from them. The burden of proof, however, lies with those 
who assert that an apparently continuous and uniform tissue can 
be separated into two parts of distinct origin. 
The evidence advanced is of two kinds—one founded on the 
comparative anatomy of stems and the other on the history of the 
tissues of the individual plant. Schoute has collected evidence to 
show that in the stems of Angiosperms a specialised layer is 
commonly distinguished from adjacent tissues either by the peculiar 
