1893 - 99 .] Edith Chick on Ricinus Communis. 
669 
This view is strengthened by a consideration and comparison of 
the vascular arrangements of (1) the root, (2) the hypocotyl, (3) 
the epicotyl of the present plant. In the root the cylinder or stele 
is the natural form in which to describe the condensed vascular 
structure. The compact arrangement is required to "withstand 
pulling strains ; and since the root, unlike the stem, directly per- 
forms an ultimate nutritive function (that of absorption), and does 
not bear organs of unlike morphological nature, its vascular 
structure is also, so to speak, ultimate , and is not made up of 
units belonging to another morphological category, as is the case 
in the stem. 
In the young hypocotyl there are distinct, widely separated 
cotyledon traces, and it is only the presence of two single layers 
of cells — the interfascicular pericycle and the endodermis — which 
allows of the term stele being applied to the vascular arrangement 
here at all; these two layers are present in response to definite 
demands on the part of the plant ; the interfascicular pericycle 
has to give rise to cambium, and the endodermis provides starch 
to be used up in its development and subsequent activity ; also in 
the thickening of the ‘bast fibres. 5 
In the epicotyl, the stele has even less individuality of its own. 
It is the result of close lateral approximation of separate leaf 
traces, each with its own cap of bast fibres ; and the slight 
approach to a perimedullary zone which is found is due to the 
contact of the small celled external conjunctive tissue which 
surrounds the xylem portion of each single bundle. 
In this region the leaf is the important thing (since the only 
function of the stem is to bear leaf structures), and the stele arises 
as a secondary phenomenon brought about by the association in 
the most obvious way of the important units the leaf traces, and 
it is the structure and course of these which provide the principal 
facts of vascular morphology. 
The fact that the cylinder so formed is continuous with that of 
the root justifies, however, the use of the term stele as applied to 
the vascular cylinder of the axis as a whole. 
There seems good reason to suppose that in the flowering plants, 
at least, the formation of a stele is generally determined by 
mechanical considerations. 
