North American Salicales . 
169 
Leaf petioles and the mid-rib of the leaf itself have such a small amount 
of vascular tissue that no rays are developed, but the vessels are surrounded 
with parenchyma. 
Other species of Salix with' terminal parenchyma and uniseriate rays 
are 5 . Hooker iana, S. japonic a, and daphnoides. 
There are a number of species with uniseriate rays and vasicentric 
parenchyma — 5 . fluviatilis , S. sitchensis , S. purpurea , 5 . lutescens, and 
5 . viminalis. Fig. 23 represents a transverse section of S. fluviatilis , 
showing vasicentric parenchyma. 
laevigata , Fig. 24, has biseriate rays and terminal parenchyma 
normally, while both 5 . sessilifolia , 5 . lasiolepis, and 5 . taxifolia have 
biseriate rays and vasicentric parenchyma. 
To sum up the genus Salix — the wood of the majority of species has 
normally uniseriate rays and terminal parenchyma. Vasicentric parenchyma 
is recalled after injury and retained in the root, in the seedling, and usually 
in the first year’s growth throughout ; multiseriate rays are recalled after 
injury and retained in connexion with leaf- and root-traces. 
Applying then the principles of comparative anatomy as enumerated 
at the beginning, it is evident that the Salicales were primitively charac- 
terized by multiseriate rays and vasicentric parenchyma. A few species retain 
these features, while others have been reduced and have lost them, except 
in certain regions especially retentive of ancestral characteristics. In con- 
sidering the relative position of different species of Popidus and Salix , 
Penhallow suggests that on the basis of its wide distribution P. balsamifera 
is the most primitive and represents an ancestral type from which many of 
the localized types have descended. In view, however, of the. anatomical 
considerations, it is evident that such forms as P. balsamifera , P. trichocarpa , 
/. tremuloides , Salix Hookeriana , S. japonic a, &c., appear to represent 
reduced types, while such forms as Popidus Fremontii and Salix sessilifolia 
represent the primitive condition. 
Professor Penhallow also regards Popidus as more primitive than Salix , 
since (p. 808) ‘the parenchyma in Popidus is always terminal’ and (p. 813) 
biseriate rayed forms are more common in Salix than in Populus. The 
first statement appears to be a mistake, P. deltoides and P. rotundifolia 
having vasicentric parenchyma, but if it were true it would only serve to 
reinforce the second in showing, not that Populus is more primitive, but 
that it is more reduced and therefore higher than Salix . On p. 820 he says 
that { the general trend of evidence so far collected, geological, geographical, 
and anatomical, is all in one direction, and that is to show that the genus 
Populus is essentially the more primitive ’. The geographical evidence 
consists in the fact that the willows are more widely distributed and less 
localized than the poplars — the latter being supposedly survivors of some 
ancestral type. This evidence, if unsupported by other more convincing 
