226 Bailey . — The Relation of the Leaf -trace to the Formation of 
families of dicotyledonous plants. The result of both investigations has 
shown that the large type of ray, normally found in living oaks, has been 
developed by the gradual aggregation and fusion of a number of small 
uniseriate rays. 
The genus Alnus offers a particularly illuminating illustration of the 
steps by which the large multiseriate ray has been * built up ’ by an aggre- 
gation and fusion of numerous smaller rays. The evidence which demon- 
strates this process of fusion occurs abundantly, both in the distribution of 
ray structures in the various species of the genus and in the life-history of 
the individual plant. 
In PI. XV, Figs, i and 2, tangential and transverse sections respectively 
of the mature wood of Alnus acuminata , H. B. K. (A. oblongifolia , Torr.), is 
shown a type of wood in which there is an entire absence of any tendency 
towards aggregation of the uniseriate rays. In marked contrast to this 
condition is that illustrated in Figs. 3 and 4, tangential and transverse sections 
of the mature wood of A. incana , (L.) Moench. In the central portion of 
the transverse section the entire absence of vessels from a band of tissue 
may be noted, and the uniseriate or linear rays are seen to be slightly 
approximated in this region. However, the tendency towards aggregation 
of the uniseriate rays is more clearly shown in the central portion of the 
tangential section, which should be examined with a pocket lens to demon- 
strate this feature more strikingly. In Figs. 5 and 6 a higher step of the 
compounding process may be seen in the mature wood of A. rubra , Bong. 
(A. Oregon a, Nutt.). Many of the aggregating rays have increased in 
size from uniseriate to bi- and tri-seriate, and are beginning to coalesce in 
places. Figs. 7 and 8 show an advanced step in the mature wood of 
A. maritima , (Marsh) Muehl. The aggregating mass has taken on the 
fusiform outline of a large oak ray, as may be seen in the tangential section, 
and is rapidly becoming a homogeneous mass of ray tissue, by the aggrega- 
tion and fusion of the enlarged uniseriate rays, as well as by the transforma- 
tion into parenchyma of fibres included in the coalescing mass of tissue. 
A completely fused aggregate or compound ray is illustrated in Fig. 9, 
a tangential section of the mature wood of A. rhombifolia , Nutt. This ray, 
upon comparison with the oak ray shown in Fig. 11, a tangential section of 
the mature wood of Quercus rubra , L., is seen to be homologous with the 
large multiseriate rays which occur in all American oaks with deciduous 
foliage. The transverse section of A. rhombifolia , shown in Fig. 10, is seen 
to illustrate a condition intermediate between those shown in Figs. 7 and 9. 
The transformation process has been completed on the left side of the 
aggregate ray, whereas the right side is still in incompleted stages of fusion. 
The elements which cross the axis of the ray diagonally are fibres in process 
of parenchymatization. This series of figures illustrates the development 
of the large compound ray from the uniseriate condition, but a great 
