No. 467.) STUDY OF THE SALICACEE. 801 
was found to be of special importance because the protoxylem 
is so localized and forms such small groups, while the transition 
elements are so scattered, that many radial sections might be 
examined without furnishing conclusive results. The diagonal 
plane of section, on the other hand, cuts through the entire 
radial extent of the transition zone and is certain to display 
whatever transition forms may be present. In this way it was 
possible to obtain the figures now presented. 
The rather scanty pith is composed of isodiametric and some- 
what thick-walled cells of large size, which, at the periphery, 
become greatly reduced in size without any diminution of the 
absolute thickness of the walls, assume the form of short cylin- 
ders, and function as resin-bearing parenchyma. There is thus 
produced a strongly resinous zone of tissue in immediate con- 
tact with the protoxylem and enclosing it on the radially inward 
face as well as the two tangential faces. It is not difficult to 
determine that the protoxylem consists primarily and chiefly of 
the usual spiral and scalariform tracheids which are common to 
the same zone in others of the Salicacez ; but a careful exami- 
nation of the diagonal section shows that there are other ele- 
ments of a transitional character not to be met with, so far as 
I am aware, in other species having an upright habit of growth. 
A somewhat .detailed account of the features noted will be 
essential. ; 
The protoxylem elements proper consist of a few spiral trache- 
ids characterized by very flat and close spirals. Such elements 
commonly have very abrupt and often transverse terminations. 
The length varies very much, $0 that while in most cases long, 
short. As exposed in sec- 
s are seen not to be sim- 
s most recently 
general effect of thi 
the appearance of a continuous series of coni 
pits. This fact also makes it clear that the spirals of Salix have 
lost the simple and primitive character of those bands which are 
to be met with in the protoxylem of the gymnosperms, or even 
in many of the vessels of the Monocotyledons, and that they 
