Editorial note — Chapter XVII

[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.]