812 THE AMERICAN NATURALIST. |. [Vor. XXXIX. 
in such types as Salir alba (Fig. 6), S. levigata, or S. longifolia 
they are so abundant as to give the impression of predomi- 
nance, an impression which is greatly strengthened in S. /asz- 
andra where the vessels justify the description of ‘strongly 
predominant throughout. The same law of distribution also 
applies to the poplars, though with the very essential difference 
that while in Salix there appears to be no very essential numeri- 
cal reduction as between the initial zone and that which termin- 
ates the growth ring (Fig. 6), in Populus on the contrary, the 
initial zone usually shows a great increase in numbers with a more 
or less gradual diminution toward the termination of growth for 
the season (Fig. 7). This is fully expressed in such species as 
P.alba, P. wislizeni, P.angustifolia, etc., but in P. monilifera 
there seems to be a predominance which is fully maintained 
until the region of the summer wood, when there is an abrupt 
reduction. 
The form of the vessel is typically oval in a radial direction 
(Fig. 7), but this is sometimes varied to oblong as in Salix alba 
(Fig. 6) or S. levigata, or to broadly oval as in Populus hetero- 
phylla, or even to transversely oval as in P. alba. While in 
general, the vessels are largest in the initial growth of the season, 
the diminution in size toward the outer face 
of the growth ring may proceed gradually 
and without any very marked alteration as in 
P. tremuloides (Fig. 7); they may become 
abruptly smaller as in P. Sremonti, or they 
may exhibit a graduated diminution as in 5. 
uva-ursi. 
One of the most striking features of the 
vessels in the transverse section, is their 
peculiar compounding, a feature also simi- 
larly expressed in the Rhamnacex. Such 

Fic, 10.—Pofulus tiremu- : : : 
loides. Radial section CO™Mpounding in the Salicacex always occurs 
showing the round or in i í : : 
oval and distant bor- such manner as to give rise to radial series 
"o (Figs. 6, 7) and in the various forms of com- 
iain imi plexity thus arising, it constitutes within cer- 
n : : 
un "Imits, a valuable differential character for the various spe- 
fies. Thus in Salir longifolia, S. discolor, S. uva-ursi, Populus 
