230 Bailey . — The Relation of the Leaf -trace to the Formation of 
vessels and the aggregation of rays, but this influence dies out radially 
after a few years’ growth, and extends only slightly above and below the 
leaf-trace. In contrast to this, in the cross-section of a small twig of A. 
iaponica , Sieb. et Zucc., seen in Fig. 18, strongly developed aggregate rays 
(marked x) extend outwards from the lateral leaf-traces and extend vertically 
many centimetres below them. Owing to this extension of the compounding 
tissue below the trace, a section at the node shows, in addition to the aggre- 
gate rays which are related to the leaf-traces of this node, other compound 
rays which are related to the lateral and median traces of higher nodes. 
One of the lateral traces illustrated in Fig. 18 is seen under higher magni- 
fication on PL XVI. Fig. 19 shows the leaf-trace bundle and associated 
tissue below the node. The band of associated tissue extends in a radial 
direction towards the inner bark, and from it vessels are gradually dis- 
appearing. Fig. 21 illustrates the same trace bundle as it passes off to the 
leaf. Vessels have entirely disappeared from the associated tissue, and the 
uniseriate rays have been gradually approximated and increased in number. 
As the leaf-trace bundle moves outward a gap is left in the protoxylem 
elements of the central cylinder. Fig. 23 illustrates the associated aggregate 
tissue which extends above the leaf-trace. The gap left by the departure 
of the leaf-trace is marked by the absence of protoxylem elements, and the 
aggregate ray extends to this, and, in consequence, apparently originates at 
the pith. A tangential view of A . japonica , showing the lateral leaf-trace 
and a small portion of the associated compounding tissue, is seen in Fig. 24. 
The associated aggregate ray is seen to be composed at this point of some- 
what loosely approximated uniseriate and biseriate rays. In serial tan- 
gential sections, cut through many subsequent annual rings, progressively 
higher stages of aggregation and fusion occur. Figs. 20 and 22 illustrate 
the relation of aggregate or compound rays to the lateral leaf-traces of 
Quercus and Cory las. Fig. 29, a transverse section of Qnercus velutina , 
Lam., shows a condition which occurs frequently in Alnus, Betnla , Carpinus y 
Ostrya , Quercus , and other genera, in which tzvo or more aggregate rays are 
related to a single lateral leaf-trace. This condition occurs very frequently 
in tissue subtending the leaf- trace, whereas above the node there occurs 
usually but a single sheet of associated tissue. 
The study of the leaf-trace in its relation to the origin and development 
of aggregate rays is very clearly shown in oak, as in this genus conditions 
exist which are extremely diagrammatic. The American Live Oak, Quercus 
virginiana , is of particular interest in this connexion, as it appears to retain 
primitive stages of the development of aggregate rays, and to indicate the 
influence which has made it advantageous for dicotyledonous plants to 
develop large storage systems in connexion with the leaf. Serial tangential 
and transverse sections, cut through the node of this oak, show that there is 
comparatively little specialized tissue above and below the leaf-trace during 
