1 70 UNITED STATES GEOLOGICAL SURVEY— TERTIARY FLORA. 



the tectli or divisions of the borders, or curving along them. Tht; areas, 

 divi(h'(l in right angle by fibrillae, form primary irregularly s(jnare meshes, 

 and the veinlets branching in various directions ultimately constitute a very 

 small polygonal areolation. One pair of slender marginal nerves jjass from 

 the top of the petiole I)elo\v the primary lateral veins, joining their branches 

 by veinlets. These general characters are, however, modiiied in many 

 ways Tlie nervation, especially, is very variable and complex, for, by the 

 addition of one or two pairs of nerves under the primary one, it becomes 

 five- or seven-palmate, while in other leaves, as for example in those of Popu- 

 Itis balsamifera var. angusllfoUa, a species especially common in the valleys 

 and along the base of the Rocky Mountains, the lower primary nerves are 

 alternate, of the same thickness as the secondary ones, all equidistant and 

 equally branching underneath, representing thus a pinnate nervation, rendered 

 remarkably similar to that of the leaves of Salix by the interposition of short 

 tertiary veins, traversing the lower side of the areas and dissolving in their 

 middle by subdivision in nervilles. In other cases, as in the so multiform 

 leaves of Populus alba, an introduced species, too common in this country, 

 the primary lateral nerves, two or three pairs, stronger and more branching 

 than the secondary ones, go straight to the |x»ints of acute lobes or of larger 

 teeth formed by expansion of the laminae, are thus craspedodroiiie, as well as 

 some of the secondary ones, and give to the leaves some of the characters 

 and the appearance of leaves of PI at anus. 



These variations in the characters of the leaves, even of the same species, 

 render their determination very difficult and somewhat unreliable; for the 

 paleontologist has rarely for examination and study a series of specimens so 

 numerous, and in such a perfect state of preservation, as are those which 

 have served for the preparation of the admirable monograph of this genus 

 in Heer's Fl. Tert. Helv., where are represented branches bearing leaves 

 of various forms, some entire, some dentate, besides buds, bracts, catkins of 

 ilowers, and seeds. 



The difficulty of determination of the leaves of Populus may, in a certain 

 degree at least, account for the great ditfercnce between the number of spe- 

 cies of this genus known as living at our time, eighteen only,* and the fossil 

 ones, of whicii Schimper's Tal. Veget. describes sixty-two, considering, how- 

 ever, nineteen of them as (h)ubtful Of the species of Poplars living at our 



"According to tbo mouograj)!! of Wesniael, in Do Caudolle's Prodiomus. 



