A SYSTEMATIC STUDY OF THE SALICACE. 
(Concluded.) 
D. P. PENHALLOW. 
ANATOMY. 
The Growth Rings. —So far as may be judged from an 
examination of fifteen species of Salix and eleven species of 
Populus, growth rings of somewhat great radial extent appear 
to be a general characteristic of the Salicacez. Such a feature 
is directly correlated with, and in fact may be taken to be a 
resultant of the very free growth for which the members of 
this family are, in general, conspicuous. Partial exceptions 
have been observed in the medium growth rings of Populus bal- 
samifera, P. monilifera, and P. heterophylla, as also of Salix 
scouleriana ; but slight deviations of this nature cannot be re- 
garded as expressing a general law, since as has been shown by 
Peirce (: 04), they may well have arisen in response to injuries 
inflicted by insects or other agencies, or they may simply be 
an expression of periodicity in climatic conditions. The same 
statement cannot be said to hold true in equal measure of Pop- 
ulus alba and P. grandidentata, since in both of these species 
the generally narrow and uniform growth rings seems to point to 
specific differences. These observations are greatly strength- 
ened by the fact that Salix uva-ursi presents a noteworthy 
deviation from the general characteristics of the family. Its 
growth rings are not only narrow, but they are variable and 
strongly eccentric. Such characteristics are very readily under- 
stood when the natural habitat of this plant is recalled. Grow- 
ing in the Alpine Garden on Mt. Washington at an elevation of 
about five thousand feet, its development is accomplished under 
the influence of a very short, season the character of which 
must vary in an exceptional manner from year to year, while the 
prostrate habit would also establish marked eccentricity of 
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