Compound Rays in the Lower Dicotyledons. 229 
in later geological times, broad rays have been built up to meet this demand 
by a process of aggregation and fusion of numerous small uniseriate rays. 
The evidence of the origin of large rays by a compounding process is 
not confined alone to the genera described above, but is amplified by 
a study of the development of ray structures in the life-history of certain 
species of Betula , Carpinus , Corylus , and Ericaceous and Rosaceous forms. 
In this connexion it should be noted that the aggregate ray, which was once 
apparently well developed, has been reduced in Castanea and certain Betu- 
laceae and Ericaceae, and occurs in general only in portions of the plant 
which are known to reflect primitive characters. Several interesting features 
of this retrograde movement will be considered by the writer in a subse- 
quent article. 
It is of special interest to study the important part that the leaf-trace has 
taken in the origin and development of the type of ray under consideration in 
this article. This feature may be seen in small twigs of certain Fagaceae and 
Ericaceae in which the compounding rays are of more or less infrequent 
occurrence. These twigs show, upon the removal of the bark, that the 
aggregate rays occur in the form of longitudinal plates of tissue extending 
above and below the minute burls which occur at the node. These burls 
are disturbances formed through several of the first-formed annual rings, 
and are produced by the lateral and median traces of the leaf. In older 
and thicker twigs the aggregate rays are more numerously developed, and 
grouped in the nodal region. Obviously, the most accurate method of 
determining the exact relation of the leaf-trace to the compounding rays is 
the examination of transverse and tangential serial sections through the 
node. The writer, adopting this method, has constructed serial sections of 
the nodes of the Cupuliferae, Ericaceae, and other families of dicotyledonous 
plants. The transverse sections were constructed starting from a point well 
below the node and passing through the node, and to a considerable distance 
above it. Similarly, tangential serial sections were constructed through 
several annual rings to the pith. By the use of this method it was seen 
that the leaf-trace, in its passage outwards to the leaf, produces a disturbing 
effect upon the surrounding woody tissue. This is expressed by a diminu- 
tion of the number of vessels and by the increase of storage tissue, particu- 
larly by the enlargement and multiplication of the rays. The extent to 
which parenchymatous tissue is developed about the entering leaf-trace 
varies with different species. In Castanea , Ostrya , and in certain alders and 
birches it is confined to the immediate vicinity of the leaf-trace, whereas in 
woods with well developed aggregate rays a large amount of storage tissue 
occurs above and particularly below the leaf- trace, as well as in radial exten- 
sion beyond it. In Fig. 17, a transverse section of a small twig of Abuts 
tenuifolia , Nutt., is illustrated the condition in which lateral leaf-traces 
(marked x) produce in their vicinity a slight tendency for the exclusion of 
