No. 467.] STUDY OF THE SALICACEE. 819 
gential section. In addition, however, the cells (2) are always 
characterized by their peculiar pits on the radial walls when 
opposite vessels. These pits are 
very rarely of the oval or quadran- 
gular form presented by Populus. 
On the other hand they are more 
or less definitely angled and crowd- 
ed as typically presented by Salix 
alba (Fig. 20.) But such a form is 
subject to very marked and sig- 
nificant variation. Thus in such 
types as S. sessilifolia, the individ- 





angled pits on the lateral walls opposite 
ual pits vary very greatly in size vessels- xa50. 
and form, sometimes being greatly 
reduced in size and multiplied in number, or again they diminish 
in number and increase greatly in size (Fig. 21); and not infre- 
quently — indeed we might say that very often, even in the same 
species — there are all degrees of transition 
exhibited from the angled pit of Salır alba 
(Fig. 20) to typical scalariform structure in 
S. sessilifolia (Fig. 22). Such transitional 
forms are peculiar to Salix. 
In the genus Populus the distribution of the 
two kinds of cells is effected in such a way that 
those of the first order occupy the central 
region, while those of the second order occupy 
the margins above and below (Fig. 16, 6, ^; 
Fig. 13, 4, 6). Occasionally there is an inter- 




Fic. 21. — Salix sessili- 
folia. Radial section 
showing a cell, a, of the 
first order ( 1), and cells, 
merging into scalari- 
form structure. X 350. 
ture, and the distinction of the two 
without any difficulty in 
spersal with the cells of the first order (Fig. 
12, 6, 6), but in such cases, as already pointed 
out, it is not always easy to distinguish the 
two kinds of cells, especially in a tangential 
section. In the genus Salix the same rela- 
tion exists (Fig. 21, a), with the difference 
that interspersal is a much more common fea- 
kinds of cells may be made 
any plane of section. The relations 
thus indicated show that there is a tendency toward replace- 
