Amongst the lowest which we may for convenience provisionally include in this group may be mentioned minute Vibrios, such as the Bacteria so much talked of in connexion with spontaneous generation, and the small plant which by its growth produces fermentation—the yeast-plant (Saccharomyces).[21] Closely allied to the yeast-plant are the "moulds" which grow on organic matters such as Penicillium, Mucor, Saprolegna, Phytophthora, the last of which is the potato disease.
A singular group of organisms goes by the name of Myxomycetes. These enigmatical creatures have been classed in turn as animals and as plants, and, indeed, at one period of their existence they seem to have more resemblance to the former, while at another stage of their life history they must unquestionably be ranked as plants. When young, they are in a semi-fluid condition, and so move that they seem, as it were, to flow over the body on which they rest. They grow upon the bark of trees or on leaves and decayed wood. They exhibit movements like those of the amæbæ and are said to engulph nutritious matters which come in their way.
The dry-looking, green, grey, red or yellow vegetable structures which encrust our rocks, walls, and trees, and which are called Lichens, form a group of plants curiously intermediate between Fungi and Algæ.
Plants somewhat higher in the scale of vegetable life are those which are termed liverworts (Hepaticæ), including the scale-mosses (Jungermanniaceæ) and Marchantia. These plants, as we shall see, are interesting on account of the variations to be found in the forms of different genera. In many, there is no stem, but only a connected series of green disk-like expansions, while others have a distinct stem with leaf-like outgrowths.
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."