DESCEIPTION OP SPECIES— SALICINE^. 165 



SALICINE^. 



SALIX, linii. 



Together with their narrowly lanceolate form, which they have in 

 common with many other plants, the leaves of Willows are recognized by 

 the following characters : — The middle nerve is strong, continued below 

 the base of the leaves into a short petiole ; the secondary veins are numerous, 

 close, parallel, generally at an open angle of divergence at or near their point 

 of union to the midrib, more oblique in coming near to the borders, where 

 they unite in continuous festoons by their curved points. These lateral veins 

 are generally intermixed with shorter tertiary ones, whose angle of divergence 

 is often different, and which, branching in the middle of the primary areas, 

 form, by subdivisions in right angle, first, large rectangular areolae, and then, 

 by multiple nervilles, a net of very small irregular meshes. As said above, 

 the form of these leaves is more generally narrowly lanceolate, more or less 

 rapidly narrowed or rounded to the petiole, but sometimes also broadly ellip- 

 tical or oblong-ovate, even ovate-subcordate. The borders of the leaves are 

 entire or simply crenate, dentate, or serrate. 



The origin of the genus seems legitimately referable to the Cretaceous 

 period. In vol. vi of the Reports of the United States Geological Survey of 

 the Territories, p. 60, pi. v, figs. 1-4, four leaves from the Dakota Group of Ne- 

 braska are described and figured as Salix proteafolia, Lesqx., whose reference 

 to the genus is so clearly indicated by the characters of form and nervation 

 that it seems indubitable. Prof Heer, whose sagacity of determination of fossil 

 leaves is so remarkable, has described, from the same formation, 6'fl/^a: nervil- 

 losa, a species of a different type of this genus; and Prof Newberry also 

 has referred to Salix, and from the same formation of Nebraska, a number of 

 leaves which he considers as representing four different species. From the 

 European Cretaceous of Blankenburg, Hartz, Dunker has described Salix 

 Hartigii, a leaf which, according to Schimper, may belong to a species of 

 Qucrcus of the section of Q. phellos; and Heer recognizes a 6ne species, 

 Salix Gcelziana, iti the formation of Quedlinburg. These are sufficient 

 iiuthorities in proof of the antiquity of this genus. Schimper remarks, in 

 Pal. Vi^get., ii, p. QQ'd, that if the attribution of certain saliciform leaves of the 

 Cretaceous formation is correct, we have, in the type of the Willows, one of 

 the most ancient forms of the subdivision of the dicotyledonous A ngiosperms. 



