No. 467.] STUDY OF THE. SALICACEE. 813 
alba, P. fremonti, P. pyramidalis, P. balsamifera, etc., the large, 
oval vessels later become reduced in size, but they chiefly 
remain single or become sparingly 2-compounded. In Salix 
alba (Fig. 6), S. sessilifolia, or S. brachystachys, the vessels are 
chiefly 1-, 2-, or rarely 3-compounded ; in S. migra, they are 
rarely 8-compounded ; in .S. /asio/epis they are chiefly single 
throughout; in S. Zancifolia they are rarely 4-compounded ; in 
Populus fremonti they are rarely 5-compounded, and similar 
variations are to be met with throughout the entire family, 
always with the status of specific characters. While the gen- 
eral facts are thus noted, it will be sufficient to refer to the 
various diagnoses for an amplification of such details. It is 
only occasionally that the vessels assume a definitely resinous 
character. This is expressed among the species so far investi- 
gated, only by the unusually resinous Sa/zr cordata, the vessels 
of which are everywhere resinous. 
Medullary Rays.—As in the Coniferales so in the Salicaceze, 
the medullary ray presents some of the most 
important of all the structural features for 
diagnostic purposes. Their value is of both 
a generic and a specific character. They are 
expressed in both radial and tangential sec- 
tions which it will be desirable to discuss 
separately somewhat in detail. 
In their tangential aspects the medullary 
rays of the Salicacee may be described as 
chiefly narrow; ı- or more rarely 2-seriate 
in part. From this there is no essential de- bordered pits. x 350. 
viation which applies to one genus more than 
to the other, except in so far as variation is rather more charac- 
teristic of Salix, and constancy more characteristic of Populus. 
It may be said that broad rays, or rays with oval or transversely 
oval cells, and 2-seriate rays are more common to the former 
than to the latter. Such differences will be made sufficiently 
clear by a comparison of Figs. 12 and 14, and Figs. 12 and 15. 
One of the chief factors in the tangential section of the ray, 
is the occurrence of ray cells of two kinds and the possibility 
of distinguishing them from one another. These cells will be 

