442 MR. F. 0. BOWER ON THE STRUCTURE OF 
consisting of five bundles, which enter the stem. In the cortex 
the lateral bundles fuse successively with the central bundle, 
which, pursuing a direct course towards the centre, finally ranges 
itself, with similar bundles from other leaves, in the central ring- 
like series, such as is normally found in Dicotyledonous stems. 
Soon after the lateral bundles of the leaf-trace become distin- 
guishable, a new system of bundles of exclusively cauline nature 
is formed in the cortex. These may be called “cortical bundles" 
or *cauline peripheral bundles" (* Rindenstránge," cf. Mettenius, 
Beitr. zur Anat. d. Cycadeen). They first make their appearance 
as groups of small cells, resulting from the division of cells of 
the cortex. The central portions of these groups develop as 
xylem, the peripheral portions as phloem, while a slowly active 
cambium lies between them. The result in each case is the 
formation of a series of wedge-shaped bundles, which, being 
arranged radially round a centre, constitute together a cylindrical 
group. The xylem, which forms the greater part of these 
bundles, consists of spiral and pitted elements, surrounded by 
and imbedded in parenchyma, which is present in considerable 
quantity; in the phloem, also, much parenchyma is present, 
together with laticiferous vessels. The course which these 
bundles take is a more or less steep spiral, according to the 
arrangement of the leaves; they run almost at right angles to 
the parastichies in which the leaves are arranged, while their 
position is such that the median bundle of each leaf is always 
about halfway between two successive cortical bundles. The 
spiral cortical bundles are themselves connected with the lateral 
bundles of each leaf-trace ; the exact point of junction varies, but 
is usually at or near the sharp curvature of the lateral bundle 
towards the central bundle of the leaf-trace, prior to their 
coalescence. The cortical bundles do not remain as single 
strands throughout their course, but here and there they sepa- 
rate into two or more strands, which soon fuse again to à single 
strand; they thus appear when isolated as a succession of loops of 
varying size (Pl. XX XVIII. fig. 4). In older stems, where the vas- 
cular system has been strongly developed, it has been possible to 
obtain vascular skeletons by the rotting of the parenchymatous 
tissues; and in these the connexions of the cauline and the 
common systems and the course of their constituent bundles 
have been more exactly observed. Part of such a skeleton is 
represented in fig. 4, in which are shown the steeply spiral 
