Two genera of aquatic plants (_Chara_ and _Nitella_) constitute another group of plants called _Characeæ_. These will be hereafter referred to both on account of peculiarities in their structure and on account of a peculiar motion of protoplasm which is easily to be seen[22] in them.
Mosses (_Musci_) are familiar objects to every one in this country, and allied to them are the so-called "club-mosses" or _Lycopods_, which form a sort of green sward in so many parts of the warmer regions of the earth. To one of the lycopods, called _Selaginella_, reference will hereafter be made in connexion with its very instructive reproductive process.
Certain humble plants, in some of which the foliage leaves present a superficial resemblance to those of a four-leaved clover, are popularly called pepperworts; by botanists, _Rhizocarpeæ_ or _Marsiliaceæ_. They are creeping or floating stemless plants which inhabit ditches or inundated places. They are scattered over both the Old and New Worlds, but are chiefly found in temperate latitudes.
The horse-tails (_Equisetaceæ_) are also found in most parts of the world, though wanting in Australia and New Zealand. They inhabit wet and sandy places, and sometimes are of a considerable size even in the present day, but in ancient geological periods they attained the proportions of trees.
This group leads us on to their allies the ferns which form a very large natural group _Filices_ or _Pteridophytes_--a group now familiar to every one interested in plants. Common as ferns are in our own country, they are far more abundant and attain to a much greater size in southern latitudes--notably in New Zealand and various Pacific islands.
All the plants hitherto enumerated, from the protococcus to the tree-ferns inclusive, together form what is commonly regarded as one great primary division or "sub-kingdom" of vegetals called CRYPTOGAMIA. In no plant belonging to this sub-kingdom--in no single cryptogam--is any flower ever developed. These form the great group which is often spoken of as "flowerless plants."
The other primary division of vegetable organisms consists of all plants with flowers, and is termed PHANEROGAMIA, and is subdivided into two sections,[23] very unequally numerous. To the first section of phanerogams--a section containing comparatively few kinds--belong all firs, pines, yews, junipers, araucarias, and a most remarkable African plant, _Welwitschia_, which has never more than two leaves, though these attain enormous dimensions. All these plants are collectively spoken of as conifers, or _Coniferæ_. Besides these, certain curious southern forms called Cycads are also associated in this section. To this section, thus composed of conifers and cycads, the name GYMNOSPERMS is given, from the naked mode of development of their young seeds. These gymnosperms are also characterized by having such peculiar and inconspicuous flowers that the ordinary observer would hardly apply that term to denote their floral organs.