232 Bailey . — The Relation of the Leaf- trace to the Formation of 
teristic lateral leaf-trace rays, originate in the early formed wood of the 
twig. In other words, when the aggregate ray becomes firmly established 
upon the plant, as in the older wood of Live and White Oak and in the early 
wood of Black Oak as well, aggregate rays which are but indirectly related 
to the leaf-traces develop to meet the demands of the increased circum- 
ference of the stem. Just as in the case of coniferous woods, new uniseriate 
rays are continually formed by the cambium, at progressively greater dis- 
tances from the pith, to maintain the proper proportion of ray tissue in the 
widening stem. 
In the development of compound rays in the Cupuliferae the lateral 
leaf-traces have played the most important part. For, although com- 
pounding is often related to the median trace or traces (three in Alnns , 
Carpinus , Corylus y and Ostrya ), the location of the bud directly over the 
median trace has apparently produced a retarding influence on the develop- 
ment of compound rays in this region. However, in the Ericaceae, where 
no lateral traces occur, the median traces have been the sole factors in 
developing the compounding rays. The fact that the Ericaceae are without 
lateral traces probably accounts in part for the primitive stages of aggrega- 
tion found in the wood rays. 
We may sum up the relation of the leaf-trace to compound rays as 
follows. In the development of the large storage systems necessary to 
plants living in regions of markedly unequal seasonal temperature charac- 
teristic of later geological time, the origin of storage tissue about the entering 
leaf-trace has proved a natural starting-point for the formation of compound 
or aggregate rays. By a gradual development of the amount of special 
storage tissue, above and below the traces, and its extension outward with 
each annual layer of growth, a larger and larger food-reserve system has 
been developed, until in the higher types, by the transformation of the com- 
pounding tissue, consisting of aggregated small rays and separating fibres, 
into ray parenchyma, homogeneous masses of ray tissue have been pro- 
duced. In less highly specialized species of the Cupuliferae the primitive 
stages of the development of aggregate rays and their relation to the leaf- 
traces are clearly shown. In specialized species, in which the aggregate ray 
is firmly established, the phases of compounding and the importance of the 
leaf-trace, as the originating influence in the development of compounding, 
have been somewhat obscured except in the younger, seedling portions of 
the plant. 
It is an interesting and important fact that in the development of the 
stem of plants which possess aggregate rays there is a marked difference 
between the general rate of growth of the woody tissues and that of the 
large aggregate rays. This is usually expressed by a strong ‘ dipping in ’ 
of the outline of the annual ring in the vicinity of the large ray. In other 
words, the compound ray produces usually a retarding influence upon 
