Other lowly plants may, at least provisionally, be placed in the great group to which mushrooms and truffles belong--the group of _Fungi_--a group the members of which agree in certain exceptional phenomena of function,[20] as well as of structure and composition--as they are exceptionally nitrogenous. 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.