« ÎnapoiContinuă »
(birch) is represented by a few species, and specimens of its peculiar bark are also common. Alnus (alder) appears in one species at least. The genus Platanus (Fig. 71), that of the plane-trees, represented at present by one
European and one American species, has several species in the Cretaceous, though the plane-trees seem to culminate in the early part of the succeeding Eocene, where there are several species with immense leaves. The large
leaves, known as Credneria, found in the Cenomanian of Europe, and those called Protophyllum (Fig. 72) in America, appear to be nearer to the plane-trees than to any others, though representing an extinct type. The laurels are represented in this age, and the American genus Sassafras, which has now only one species, has not one merely but several species in the Cretaceous. Diospyros, the persimmon-tree, was also a Cretaceous genus.
FIG. 72.-Protophyllum boreale, Dawson, reduced. Upper Cretaceous, Canada.
The single species of the beautiful Liriodendron, or tuliptree, is a remnant of a genus which had several Cretaceous species (Figs. 74, 75). The magnolias, still well represented in the American flora, were equally plentiful in the Cretaceous (Fig. 73). The walnut family were well represented by species of Juglans (butternut) and Carya, or hickory. In all, no less than forty-eight genera are present belonging to at least twenty-five families, running through the whole range of the dicotyledonous exogens. This is a remarkable result, indicating a sudden profusion
of forms of these plants of a very striking character. It is further to be observed that some of the genera have
FIG. 73.-Magnolia magnifica, Dawson, reduced. Upper Cretaceous, Canada.
many species in the Cretaceous and dwindle toward the modern. In others the reverse is the case they have expanded in modern times. In a number there seems to have been little change.
Dr. Newberry has given, in the "Bulletin of the Torrey Botanical Club," an interesting résumé of the history of the beautiful Liriodendron, or tulip-tree, which may be taken as an example of a genus which has gone down in importance in the course of its geological history.
"The genus Liriodendron, as all botanists know, is represented in the present flora by a single species, the tulip-tree,' which is confined to eastern America, but grows over all the area lying between the Lakes and the Gulf, the Mississippi and the Atlantic. It is a magnificent tree, on the
whole, the finest in our forests. Its cylindrical trunk, sometimes ten feet in diameter, carries it beyond all its associates in size, while the beauty of its glossy, lyreshaped leaves and tuliplike flowers is only surpassed by the flowers and foliage of its first cousin, Magnolia grandiflora. That a plant so splendid.
FIG. 74.-Liriodendron Meekii,
FIG. 75.-Liriodendron primævum,
should stand quite alone in the vegetation of the present day excited the wonder of the earlier botanists, but the sassafras, the sweet-gum, and the great Sequoias of the far West afford similar examples of isolation, and the latter are still more striking illustrations of solitary grandeur.' (Figs. 74 and 75.)
"Three species of Liriodendron are indicated by leaves found in the Amboy clays-Middle Cretaceous-of New Jersey, and others have been obtained from the Dakota group in the West, and from the Upper Cretaceous strata of Greenland. Though differing considerably among themselves in size and form, all these have the deep sinus of the upper extremity so characteristic of the genus, and the nervation is also essentially the same. Hence, we must conclude that the genus Liriodendron, now rep
resented by a single species, was in the Cretaceous age much more largely developed, having many species, and those scattered throughout many lands. In the Tertiary age the genus continued to exist, but the species seem to have been reduced to one, which is hardly to be distinguished from that now living. In many parts of Europe leaves of the tulip-tree have been found, and it extended as far south as Italy. Its presence there was first made known by Unger, in his 'Synopsis,' page 232, and in his 'Genera et Species,' page 443, where he describes it under the name of Liriodendron procaccinii. The genus has also been noticed in Europe by Massalongo, Heer, and Ettingshausen, and three species have been distinguished. All these are, however, so much like the living species that they should probably be united with it. We here have a striking illustration of the wide distribution of a species which has retained its characters both of fruit and leaf quite unchanged through long migrations and an enormous lapse of time.
"In Europe the tulip-tree, like many of its American associates, seems to have been destroyed by the cold of the Ice period, the Mediterranean cutting off its retreat, but in America it migrated southward over the southern extension of the continent and returned northward again with the amelioration of the climate."
Leaves of Liriodendron have been recognised in the Cretaceous of Greenland, though it is now a tree of the warm temperate region, and Lesquereux describes several species from the Dakota group. But the genus has not yet been recognised in the Laramie or in the Upper Cretaceous of British Columbia. In the paper above quoted, Newberry describes three new species from the Amboy clays, one of which he considers identical with a Greenland form referred by Heer to L. Meeki of the Dakota group. Thus, if all Lesquereux's species are to be accepted, the genus begins