[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.

It follows from the above, that the Lower and Upper Cretaceous rocks are, from a botanical point of view, sharply separated from one another.  The Palaeozoic period, as we have seen, is characterised by the prevalance of “Flowerless” plants (Cryptogams), its higher vegetation consisting almost exclusively of Conifers.  The Mesozoic period, as a whole, is characterised by the prevalence of the Cryptogamic group of the Ferns, and the Gymnospermic groups of the Conifers and the Cycads.  Up to the close of the Lower Cretaceous, no Angiospermous Exogens are certainly known to have existed, and Monocotyledonous plants or Endogens are very poorly represented.  With the Upper Cretaceous, however, a new era of plant-life, of which our present is but the culmination, commenced, with a great and apparently sudden development of new forms.  In place of the Ferns, Cycads, and Conifers of the earlier Mesozoic deposits, we have now an astonishingly large number of true Angiospermous Exogens, many of them belonging to existing types; and along with these are various Monocotyledonous plants, including the first examples of the great and important group of the Palms.  It is thus a matter of interest to reflect that plants closely related to those now inhabiting the earth, were in existence at a time when the ocean was tenanted by Ammonites and Belemnites, and when land and sea and air were peopled by the extraordinary extinct Reptiles of the Mesozoic period.