The plant-remains of the Lower Cretaceous greatly resemble those of the Jurassic period, consisting mainly of Ferns, Cycads, and Conifers. The Upper Cretaceous rocks, however, both in Europe and in North America, have yielded an abundant flora which resembles the existing vegetation of the globe in consisting mainly of Angiospermous Exogens and of Monocotyledons.[23] In Europe the plant-remains in question have been found chiefly in certain sands in the neighbourhood of Aix-la-Chapelle, and they consist of numerous Ferns, Conifers (such as _Cycadopteris_), Screw Pines (_Pandanus_), Oaks (_Quercus_), Walnut (_Juglans_), Fig (_Ficus_), and many _Proteaceoe_, some of which are referred to existing genera (_Dryandra, Banksia, Grevillea_, &c.) [Footnote 23: The "Flowering plants" are divided into the two great groups of the Endogens and Exogens. The _Endogens_ (such as Grasses, Palms, Lilies, &c.) have no true bark, nor rings of growth, and the stem is said to be "endogenous;" the young plant also possesses but a single seed-leaf or "cotyledon." Hence these plants are often simply called "_Monocotyledons_." The _Exogens_, on the other hand, have a true bark; and the stem increases by annual additions to the outside, so that rings of growth are produced. The young plant has two seed-leaves or "cotyledons," and these plants are therefore called "_Dicotyledons_." Amongst the Exogens, the Pines (_Conifers_) and the Cycads have seeds which are unprotected by a seed-vessel, and they are therefore called "_Gymnosperms_." All the other Exogens, including the ordinary trees, shrubs, and flowering plants, have the seeds enclosed in a seed-vessel, and are therefore called "_Angiosperms_." The derivation of these terms will be found in the Glossary at the end of the volume.] In North America, the Cretaceous strata of New Jersey, Alabama, Nebraska, Kansas, &c., have yielded the remains of numerous plants, many of which belong to existing genera. Amongst these may be mentioned Tulip-trees (_Liriodendron_), Sassafras (fig. 186), Oaks (_Quercus_), Beeches (_Fagus_), Plane-trees (_Platanus_), Alders (_Alnus_), Dog-wood (_Cornus_), Willows (_Salix_), Poplars (_Populus_), Cypresses (_Cupressus_), Bald Cypresses (_Taxodium_), Magnolias, &c. Besides these, however, there occur other forms which have now entirely disappeared from North America--as, for example, species of _Cinnamomum_ and _Araucaria_.