CHAEACTEBS 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 clasping 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 in one 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 difficulty 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 except for the nerves which 

 project heavily, at least toward the base. How far the fleshiness of 

 both stem and leaf varies in fresh material can 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 structure, as will the very heavy wrinkling of the 

 stem in P. fragile and other species and its uniform fine cross-striation 

 in P. Fid and one or two others. 



