[DN] With reference to this, something may be learned from the leaves of modern trees. In these, young shoots have leaves often less toothed and serrated than those of the adult tree. A remarkable instance is the _Populus grandidentatus_ of America, the young shoots of which have entire leaves, quite unlike except in venation those of the parent tree, and having an aspect very similar to that of the Cretaceous poplars. [Illustration: Fig. 71.--_Platanus nobilis_, Newberry, variety _basilobata_. Laramie. Much reduced.] [Illustration: Fig. 72.--_Protophyllum boreale_, Dawson, reduced. Upper Cretaceous, Canada.] The single species of the beautiful _Liriodendron_, or tulip-tree, 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 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. [Illustration: Fig. 73.--_Magnolia magnifica_, Dawson, reduced. Upper Cretaceous, Canada.] 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, lyre-shaped leaves and tulip-like flowers is only surpassed by the flowers and foliage of its first cousin, _Magnolia grandiflora_. That a plant so splendid 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.) [Illustration: Fig. 74.--_Liriodendron Meekii_, Heer. (After Lesquereux.)] [Illustration: Fig. 75.--_Liriodendron primævum_, Newberry. (After Newberry.)]