No. 467 STUDY OF THE SALICACEE. 799 
be inferred from the observed structural conditions, that in the 
Salicacex as a whole, the bark expands or ruptures in such a 
way as to provide a minimum of pressure which is practically 
uniform during the entire period of growth. 
The Transition Zone — The protoxylem region in the Sali- 
cacex, or what may with more propriety be termed the ¢ran- 
sition zone in accordance with the previous use of that term 
(Penhallow, : oo, : 04c), is exclusively confined to the growth of 
the first year in accordance with the general law of growth 
which governs the Dicotyledonous angiosperms, and the follow- 
ing observations must therefore be interpreted in that sense. 
So far as our studies have gone, they show that in the arbo- 
rescent Salicaceze, whether Populus or Salix, the transition zone 
is narrow and that it consists almost exclusively of a few spiral 
and scalariform elements. I have as yet been unable to discover 
in such forms, any extension of the protoxylem which would 
admit of transitional forms of the elements. The transition from 
the primitive protoxylem elements on the one hand, to com- 
pleted forms of wood cells without pits, and of broad vessels 
with hexagonal pits on the other hand, appears to be developed 
with great abruptness. These features appear to be so well 
defined and constant as to make it apparent that the transition 
zone has been so far reduced as to approach the point of actual 
extinction. 
In 1897 an opportunity was presented to collect some stems 
of Salir uva-ursi from the Alpine Garden, Mt. Washington. 
At that elevation the greatly reduced shrubs are forced to 
assume a prostrate habit of growth, in consequence of which 
the rather slender and somewhat sinuous stems are buried un- 
der a considerable accumulation of plant débris. In such sit- 
uations, although there is usually an abundance of water during 
the summer, the plant is of a somewhat xerophilous habit as 
made evident by the reduced and somewhat coriaceous foliage, 
in obedience to the low temperature and the consequent inability 
of the roots to provide for an actively moving transpiration WE 
Schimper (: 03). This xerophilous habit 
ral structure of the wood as may 
osis, from which it will be 
rent, as pointed out by 
is also expressed in the gene ; 
be observed by reference to the diagn 
