234 Bailey . — The Relation of the Leaf -trace to the Formation of 
at the left is at first feebly developed, and the segment is in consequence 
but slightly depressed upon the left side. In addition it should be noted 
in this figure that the segment becomes more strongly depressed as the 
aggregate rays become more firmly established in the older portion of 
the stem. 
From this we see that the aggregate ray, as it possesses a different rate 
of growth from the rest of the secondary wood, has often a marked effect 
upon the development of the stem. This is expressed in most cases by 
a retarding influence upon growth, which produces a distinct sag in the out- 
line of the annual rings in the vicinity of the large ray. The most striking 
illustrations of the effect of this retarding influence of the compound ray are 
seen in the fluted stem of the Blue Beech and in the depressed small segments 
of oak branches. 
Conclusions. 
The origin and development of aggregate or compound rays, as revealed 
by the study of the comparative anatomy and morphology of the Betulaceae 
and higher Cupuliferae, and by the phylo- 
genetic evidence afforded by seedling plants 
and traumatic areas, is significant in a con- 
sideration of the development of the fibro- 
vascular cylinder of woody plants. The 
theory of the origin and development of 
the central cylinder of Gymnosperms and 
Angiosperms has remained practically un- 
changed, in the botanical literature of the 
Old and New World, since the publication 
of Sachs’ ‘ Lehrbuch der Botanik \ Subse- 
quent authoritative writers have adopted 
the theory, and the Sachsian figures illus- 
trating it have been largely copied by 
European writers, as well as by American 
writers. This conception, which appears to 
have really originated with Sanio and to 
have been exploited by Sachs and De Bary, 
may be summed up as follows, using the 
Sachsian figures as illustrations. In her- 
baceous plants and in the £ embryonic stem * of woody plants (see a), 
there is developed a ring of primary collateral bundles. These are 
arranged in a circle, thus dividing the fundamental tissue into two portions, 
a pith M and a cortex R. Each open bundle possesses primary phloem,/, 
on the outside and primary xylem, ;r, on the inside, and these are separated 
by a meristematic tissue or fascicular cambium. In the development of 
Text-fig. — Description in text. 
