Another important group of the _Thallophytes_ is formed by the _Fungi_, which include the familiar mushrooms, toadstools, and the sap-balls so commonly seen on decaying trees; also the smaller forms known as moulds, mildew, and smut. These, also, are entirely cellular in structure; and, since they develop no chlorophyll, are compelled to live as parasites on living beings or to derive their food from decaying organic matter. Thus they are the creatures of corruption, their presence always denoting the breaking down of living matter or of matter that has previously lived. Now leaving the _Thallophytes_, and passing over the small group of aquatic plants known as the _Charales_, we come to the _Muscineæ_, which contains the Liverworts (_Hepaticæ_) and the Mosses (_Musci_). The plants of both these groups require much moisture, and are found principally in damp, shady situations. Like the preceding groups they are cellular in structure, never producing true vascular bundles such as the higher plants possess; and their life histories are rendered interesting by the ‘alternation of generations’ which they exhibit. The first generation is a sexual one produced from the spores, and consists either of a mass of delicate threads from which a plant with a leafy axis is developed by a process of budding, or of a little green frond (the _thallus_). These bear the male and female elements, called respectively the _antheridia_ and the _archegonia_; and when the central cells of the latter are fertilised by the former, they give rise to a case, with or without a stalk, containing a number of spores. When the case is ripe, it opens horizontally by means of a lid, thus liberating the spores. Following these in the ascending scale are the _Vascular Cryptogams_, in which some of the cells become modified into true vessels. Here, too, the plants exhibit a distinct alternation of generations, the spore first giving rise to a small, leafless body, the _prothallium_, which bears the sexual organs; and then the female elements, after fertilisation, produce the spore-bearing plant. This group contains quite a variety of beautiful and interesting plants, including the Ferns (_Filicales_), Horsetails (_Equisetales_), Club-mosses (_Lycopodiales_), Water Ferns (_Rhizocarpeæ_), and _Selaginellales_.