Compound Rays in the Lower Dicotyledons. 233 
growth in its vicinity. This condition is shown in the cross-sections illus- 
trated in Figs. 4, 6, 8, and 10, and it may be noted in addition that the sag 
in the outline of the year’s growth increases pari passu with the develop- 
ment of the ray. With higher phases of compounding and the increase in 
size of the ray, the * dipping in ’ of the compound ray becomes more 
strongly developed. This retarding influence of the aggregate ray upon 
the growth of the stem is well marked in the twigs of the Blue Beech (Car- 
pinus caroliniana , Walt.) and Betula pumila , L. When the bark is removed 
from small twigs of these plants it is seen that the woody cylinder appears 
strongly crenulated in outline. The aggregate rays in these species are of 
large size and strongly developed, and produce pronounced sags in the 
periphery of the stem. Individually the aggregate rays produce a retarding 
influence upon growth, and when approximated several aggregate rays pro- 
duce a general dip in the outline of the stem by their co-ordinated retarding 
influence upon growth. This condition is well illustrated by the bole of the 
Blue Beech {Carpinus caroliniana). As is well known, this small tree is 
characterized by a strongly fluted stem. A cross-section of such a stem 
reveals the interesting fact that the furrows in the stem correspond to bands 
of numerous closely approximated compound rays, and«the ridges to areas 
in which the rays are nearly absent. The stem of other members of the 
Betulaceae and of Quercus and Fagus often show a similar condition. 
Where the compound rays are approximated there is a corresponding 
depression in the outline of the stem. The most striking illustration of the 
retarding influence of aggregate rays upon the development of the woody 
cylinder occurs in the branches of White and Red Oaks, and in Clematis. As 
may be seen in Fig. 25, the stem appears to be divided into ten segments, 
five of which are larger wedge-shaped segments projecting beyond five 
smaller depressed segments with parallel sides. The sunken segments, as 
will be noted on closer inspection, are separated from the larger ones by the 
lateral leaf-trace rays which have been described earlier in this article. In 
fact, it is the strong retarding influence of this storage tissue, associated 
with the lateral leaf-traces and longitudinally relayed from node to node in 
ten vertical lines up the stem, which has produced the depression repre- 
senting the smaller segments. Where the rays are strongly developed the 
segments are strongly depressed, and when the aggregate rays are feebly 
developed the segments are correspondingly slightly depressed. Similarly, 
when the ray on one side of a small section is slightly developed or absent, 
the segment is seen to be unsymmetrically developed. Thus in Fig. 25 one 
of the small segments is depressed upon the right side by the aggregate ray 
(marked a), whereas on the opposite side (£), owing to the absence of a com- 
pound ray, the segment rounds up to the outline of the larger segment 
without well-marked sags in the annual rings, such as occur on the opposite 
side. A similar condition is shown in the lower portion of Fig. 27 ; the ray 
