508 T ho day. — On the Organization of Groivth and 
tudinally . 1 So long as a leaf is growing, its influence is felt throughout the 
bundles of its leaf-trace and the synthetic traces with which these are con- 
tinuous below. The synthetic traces come under the combined influence of 
a continual succession of expanding leaves. The upper part of a leaf-trace, 
on the other hand, follows the lead of its own leaf, and its growth practically 
ceases when the leaf is fully expanded. The lower part commences to form 
secondary xylem under the influence of its own leaf, but this change from 
primary to secondary growth apparently involves a transference from the 
sphere of influence of the leaf into that of the apex. After the growth of 
the leaf has ceased the influence of the apex gradually encroaches still farther 
upwards until it controls the activity of the cambium throughout the bundle. 
It seems necessary also to suppose that the sphere of influence of the 
leaf extends to the adjoining parenchyma of the medullary rays that flank 
the trace bundles and inhibits meristematic activity there ; for otherwise it 
is difficult to understand the fact that, in spite of the active extension of the 
cambium from the synthetic trace on the other side of a ray, the complete 
bridging of the ray by cambium, and still more by secondary wood, is so 
long delayed. This assumption gives a more precise meaning to the 
localization of the leaf-trace bundles in this multifasciculate stem, and also 
covers the persistence of the primary medullary rays which is a necessary 
condition of its specialized tangential mode of growth in thickness. 
The view of the observed correlations here tentatively outlined is at 
least a useful working hypothesis. It suggests a field for experimental 
investigation in the study of the effects of removing leaves, at various 
stages of their development, on the growth of the trace bundles at different 
levels. 
In conclusion, it may be pointed out that plants were intentionally 
selected for their freedom from lateral branches, which would have added 
further complications. A study of the way in which branches are accommo- 
dated and their influence on the main stem in this plant would not be 
without interest. 
The only other species which has been examined for comparison is 
Helianthus tuberosus . The aerial stem of this species shows similar 
tangential growth, similarly distributed. 
As regards the cambial origin of the primary xylem, this is probably 
a widespread phenomenon among Angiosperms. The absence of a sharp 
line of demarcation between primary and secondary wood has often been 
remarked upon. A casual glance at the first annual ring of the Oak and 
1 If the correlations on which Miss Saunders has based her ‘ Leaf-skin Theory ’ (Ann. Bot, 
xxxvi, p. 135, 1922) are to be similarly interpreted, they imply a delimitation of spheres of 
influence in the superficial tissues on a plan very different from that which governs their delimitation 
in the vascular zone. 
