CHARACTERS—LEAF 9 
others, like P. affine, the leaves are technically sessile; but in the greater 
number the leaf is attenuated into what almost equally well may be 
called a winged petiole or a subsessile base. In one group only, that of 
P. amplexicaule, the base or petiole is dilated or elasping at its insertion. 
Comparison of two such species as P. flavens and P. racemosum 
reveals a marked contrast in the position of the principal nerves which 
are found in all of the larger leaves in this genus,—the former being 
distinctly basinerved, and the latter pinnately veined. Occasionally, as 
in P. chrysocladon, the stronger of the nerves іп опе of the former group 
gives off a few fine or indistinct lateral veins: in P. Eggersii, P. peruvi- 
anum and a few others the midrib, strong and heavily branched below, 
disappears below the middle of the leaf; and in a very few cases the 
vascular group of the petiole continues for a short distance into the 
expanded blade before breaking into its component bundles to form the 
nerves of the leaf. 
Great differences are found in leaf texture and venation, but as a 
rule species that are closely related differ little in this respect. The 
terminalogy of such characters when observed in the herbarium is likely 
to prove misleading when applied to fresh material, but however it 
may be corrected to suit the latter it will always prove necessary when 
future collections are compared with the types to which they correspond. 
As an illustration may be cited our common eastern mistletoe, P. flaves- 
cens, which possesses rather fleshy leaves with heavy somewhat raised 
nerves when fresh or wilting, but appears subcoriaceous in the herbar- 
ium ; while the related P. villosum of the West Coast is rather coriaceous 
even when fresh. As examples of some of the extremes in this respect 
shown by herbarium material, may be noted P. undulatum and the 
aggregate of forms commonly called P. latifolium, in which a heavily 
raised midrib is seen beneath, while some diffieulty may be found in dis- 
tinguishing its pinnately placed branches: P. robustissimum, in which 
the nervation is very inconspicuous in the opaque closely papillate 
leaves: the group of Andean species which I am calling ‘‘ Andinae’’, 
in which the leaf dries thin with sharply raised fine nerves: and P. flavens 
and its allies, which are finely nerved and venulose and wrinkled above, 
while beneath they are smooth and dull exeept for the nerves which 
projeet heavily, at least toward the base. How far the fleshiness of 
both stem and leaf varies in fresh material ean be inferred only for 
most species; but the rugulose upper surface of the leaves in those last 
mentioned and in the Brazilian P. chrysocladon no doubt will find 
ready explanation in strueture, as will the very heavy wrinkling of the 
stem in P. fragile and other species and its uniform fine eross-striation 
in P. Fici and one or two others. 
