170 Holden . — Reduction and Reversion in the 
testimony, is of little weight. The geological evidence consists in the fact 
that Popidus primaeva is supposed to be older than any Salix , but the 
finding of both Saliciphyllum and Populiphylliim in the Potomac discredits 
any conclusions from fossil evidence. As for the anatomical evidence, there 
is only one species of Populus with biseriate rays, and there are only two 
with vasicentric parenchyma, as opposed to a dozen or more with terminal 
parenchyma and uniseriate rays. On the other hand, there are more species 
of Salix with vasicentric than with terminal parenchyma, and almost as 
many with biseriate as uniseriate rays. Accordingly, since both Salix and 
Popidus , as shown by comparative anatomical evidence, are descended from 
forms with multiseriate rays and vasicentric parenchyma, it seems clear 
that Popidus is more reduced than Salix and consequently higher. 
The geographic distribution of the different species is of interest. The 
three North American species of Salix with vasicentric parenchyma and 
biseriate rays are indigenous west of the Mississippi, and the one species of 
Popidus with biseriate rays is a native of California. On the other hand, 
the more reduced forms with uniseriate rays and terminal parenchyma 
grow in the eastern as well as the western part of the country. It is not 
uncommon to find that the western species of the United States are more 
primitive and less reduced than the eastern. For example, Mr. Bailey has 
recently investigated the western chestnut, Castanopsis , and finds here various 
structures, present in a greatly reduced form in the eastern Castanea . 
Having discussed the anatomy of the Salicaceae, it is now desirable 
to discuss its bearing on their systematic position. Recent investigations 
in wood structure have shown that the most constant feature for diagnostic 
purposes is the distribution of the wood parenchyma, and the next best 
the width of the rays. The evolution of rays in the vascular plant series 
would be somewhat as follows : most Gymnosperms have uniseriate rays. 
In the epicotyledonary seedling stem of Quercus there are uniseriate rays 
near the pith which on passing out in certain segments become bi- and tri- 
seriate ; then by approximation of the rays and elimination of vessels these 
segments become ‘ false ’ rays — the permanent condition in the adult Live 
Oak. Further on, by the elimination of the intervening tracheides the rays 
become solidly parenchymatous, as represented in the homogeneous masses 
of ray parenchyma in the mature wood of North American White and Black 
Oaks . 1 That rays of this type are made up of compounded multiseriate 
rays is proved by the wound reaction . 2 In genera slightly higher than 
Quercus , i. e. Cary a, Ostrya y Fagus , &c., these rays, solid at the pith, break 
up as they pass out, until by the intrusion of fibres and vessels in both 
1 Eames, A. J. : On the Origin of the Broad Ray in Oaks. The Botanical Gazette, xlix, 
pp. 161-7, No. 3 - 
2 Bailey, I. W. : Reversionary Characters of Traumatic Oak Woods. The Botanical Gazette, 
b PP- 374-8°; No. 5. 
