238 Bailey .- — The Relation of the Leaf- trace to the Formation of 
in the dissection of the woody cylinder. By the development of these 
structures the woody cylinder becomes at first a network of xylem tissue 
filled in by large thin sheets of storage tissue. A higher step of this 
dissection process, in the Cupuliferae, may be seen in the mature twigs 
of Red and White Oak. In these highly specialized and diagrammatic stems, 
as also in Clematis , the aggregate rays formed by the leaf-traces at each 
node are relayed from node to node up the stem in long vertical lines. In 
these stems, therefore, the woody cylinder is actually divided into ten 
so-called fascicular segments, separated by sheets of vertically fused aggre- 
gate rays or interfascicular segments so called. Owing to the fact that the 
large rays are grouped in approximated pairs, five smaller segments are 
depressed by the concentrated retarding influence of these rays upon their 
growth. Among higher families of the Dicotyledons there exist still higher 
degrees of the dissection of the central cylinder by means of the parenchyma- 
tization of secondary xylem, in relation to the leaf. This expresses itself in 
the transformation of larger and larger segments of the central cylinder into 
parenchyma, and in the gradual decrease in thickness of the woody cylinder. 
The cylinder is thus split up into small segments or bundles, separated 
by broad radial stripes of parenchyma subtended by the so-called inter- 
fascicular cambium. In the progress of the herbaceous habit the interfasci- 
cular cambium also loses its activity. Eames, in an article which appears 
with this, has succeeded in demonstrating these progressive transitions in 
certain Rosaceous species. Thus, for example, the prostrate biennial or 
perennial stems of Potentilla palustris, as well as the seedling plant, possess 
an unbroken central cylinder, whereas the cylinder of the erect annual stem 
a short distance above the rhizome breaks up into a typically herbaceous 
stem. 
From this consideration of the comparative anatomy of living and 
fossil plants, particularly of the Cupuliferae, of the phylogenetic significance 
of seedling plants, and of the origin and development of storage tissue in 
relation to the leaf-trace, we come to the conclusion that the Sachsian 
hypothesis of the origin of the central cylinder of woody plants, based upon 
appearances rather than upon an adequate study of anatomical facts, must 
be reversed in order to agree with actual conditions among the higher seed 
plants. The most striking feature of this study has been t'he important 
part that the leaf-trace has played in the development of complex ray 
structures, and in the development of the herbaceous habit. 
In view of the confused terminology of ray structures which exists in 
botanical literature, it seems desirable at this point to endeavour to unravel 
this tangle in the light of recent investigation upon ray structure. All ray 
structures occurring in the xylem portions of plants have been commonly 
called ‘ medullary rays ’. Inasmuch as these structures originate only 
with secondary growth, and are in no sense related to fundamental tissue, 
