ioo8 
Thompson.—On the Origin of the 
be a matter of course. Nevertheless, one still finds the breaking-up process 
taking place at the leaf-trace. PI. LXXVII, Fig. 7, which is a photograph 
of a section above the node in Vaccinium corymbosnm , shows a broad ray at 
the bottom being dissected into the numerous multiseriate ones seen at the 
top. In addition to these, other multiseriate rays are visible at the left and 
right which are quite independent of the broad ray. In Vaccinium corym - 
bosum y therefore, the breaking up of the broad ray at the leaf-trace takes place 
as previously described, and in addition new rays arise quite independently 
of the leaf-trace. The adult wood will therefore be characterized by a 
rather uniform distribution of multiseriate rays, although here and there 
one may observe a slight grouping, the result of the breaking up of 
a compound ray. 
A tangential view of the condition existing in Vaccinium corymbosum 
is given in Fig. 8. The leaf-trace is visible at the extreme lower edge of 
the figure, and the ray in the process of dissection above it. Independently 
arising multiseriate rays are to be seen at the left and right. 
A similar condition is present in Gaylussacia resinosa , except that the 
solid type of ray is more evanescent. 
It is obvious that the net result of the process of dissection of com¬ 
pound rays and the fixation on the cambium of the habit of forming the 
dissected type will be the uniform distribution of the latter type throughout 
the wood. Such a condition one finds to exist in many species of the 
Ericaceae; for example, Ctethra alnifolia (L.) (Fig. 9). At the bottom is 
the pith, and throughout the wood are multiseriate rays intermixed with 
uniseriate ones. Fig. 10 shows the condition in tangential section, the 
multiseriate rays being strikingly uniform in distribution. Furthermore it 
is important to notice that the leaf-trace at the bottom of the figure is no 
longer accompanied by a compound ray or by an aggregation of multi¬ 
seriate rays. This plant presents, therefore, the completely developed 
condition of the multiseriate ray of the Dicotyledons. 
Among other Ericaceous plants presenting equally well developed 
conditions may be mentioned Andromeda Jloribunda (Pursh), Lyonia 
ligustrina L. (DC.), Leucothoe racemosa L. (Gray). 
To summarize the conditions found in the Ericaceae : The breaking 
up of compound rays into smaller multiseriate ones is of common occur¬ 
rence. In some species only one such group may be seen in a given 
section. Then by the occurrence of several traces at the same level, by 
the overlapping of rays from traces at different levels, by the vertical 
extension of groups of rays with the increase in size of the stem, and above 
all by the acquired habit of forming multiseriate rays independently of the 
leaf-trace, the multiseriate rays become uniformly distributed throughout 
the stem, and the dissection process is lost. 
Turning now to other groups of Dicotyledons in which the compound 
