670 Proceedings of Royal Society of Edinburgh. [sess. 
It occurs in the root as a well-defined cylindrical axial strand, 
apparently for the purpose of resisting pulling strains. In the 
stem it is required for support and the resistance to bending strains, 
and hence the formation of fibrous pericycle and sometimes the 
sclerization of the rays and perimedullary zone ; when this takes 
place we have the stele at its highest point of development. 
On the other hand, the primary function of the vascular system 
is conduction, and this is carried on by the leaf traces whether a 
stele is present or not. 
In conclusion, I wish to express my extreme indebtedness to 
Mr A. G. Tansley for the valuable advice and assistance he has 
given me in the preparation of this paper. 
EXPLANATION OF PLATES. 
Plate I. 
Ext. c., external conjunctive ; X., xylem ; P., phloem. 
Figs. 1-6. Diagrams, drawn to scale, representing the transition from 
root to hypocotyl ; fig. 1 showing cortex ; figs. 2-6 limited by endodermis. 
Figs. 7, 7 (a), 7 {b). Diagrams illustrating gamostely in an abnormal root. 
All three figures are limited by the endodermis. 
Fig. 7. Showing two normal root steles inclosed by a common endodermis 
{end . ). 
Fig. 7 (a). Vascular cylinder, roughly crescent-shaped In transverse section, 
internal endodermis becoming less distinct ; inclosed cortex {cor.). 
Fig. 7 {b). After much shifting the xylems and phloems are arranged 
symmetrically. 
Also figs. 16a-21. 
Plate II. 
Fig. 8. Diagram illustrating the course of bundles in the Hypocotyl from 
12 cm. to 19 '2 cm. above the ‘collet’ in a given seedling. The vascular 
cylinder unrolled, and looked at from the inside. 1, 2, 3, 4, 5, 6, 7, 8, 
cotyledon traces ; a, c, d , b, f, e, trunks by the union of epicotyledonary leaf 
traces, and each connecting with adjacent cotyledon traces by two divergent 
shanks ; a, j8, leaf traces in vertical plane passing through the cotyledons, 
which join on to the cotyledon traces at a higher level than the shanks of a, 
b, c, etc. ; at their upper limit they pass off to axillary buds in the axils of 
the cotyledons. 
Figs. 9-14. Diagrams of transverse sections of hypocotyl at different levels. 
Fig. 9. At 12-5 cm., from ‘collet’ showing 8 cotyledon traces. 
Fig. 10. At 13 cm., from ‘collet’ showing 8 cotyledon traces, and the 
junction of the shanks of a , b, c , d, etc., with them. 
Fig. 11. At 14 ’25 cm., showing 8 cotyledon traces and the shanks of a, b, 
c, d t e , /. 
