TURALIST. [Vor. XXXIX. 
816 THE AMERICAN NATURALIST. | [VoL 
it will be found that where such differences exist, the is a d 
second order are generally narrower and relatively hé : esc 
longer. The cells of the first order are, as a rule, C K de iis 
and oblong. Few exceptions to this occur (Fig. 15) t ies iee 
may appear not only in the same species but also in bee 
section. On the whole it may be said that Populus is ge Bn 
characterized by narrow and oblong cells — the tendency Be 
ovalor round form being of the nature of an exception. Re 
genus Salix, on the contrary, the typical form of the ce ub 
oval, round or even transversely oval (Figs. 14, 15), the o : 
FB of narrow and oblong cells being of the nature of an excep 



Fic, 16,.— Populus tremuloides. Radial 
: inous 
section of medullary rays showing the often resino! 
character and Structural details in c 
50. 
ells of the first order, 2, and the second order, 5. X 
an; nger 
tion. The cells of the second order (2) are also much ee 
and narrower than in Populus, differences which will be pod 
explained by the radial. sections, so that considering these 
à i eric 
ences as a whole, they again afford a valuable element in gen 
differentiations, 
f specific value. These differences 
prominently in the form and character of the 
necessary to recognize cells of two kinds. 
ray is composed of cylindrical cells several 
are expressed most 
cells whereby it is 
Fundamentally the 
