Of the first, the puff-balls (Fig. 49) are common examples. One species, the giant puff-ball (_Lycoperdon giganteum_), often reaches a diameter of thirty to forty centimetres. The earth stars (_Geaster_) have a double covering to the spore fruit, the outer one splitting at maturity into strips (Fig. 49, _B_). Another pretty and common form is the little birds'-nest fungus (_Cyathus_), growing on rotten wood or soil containing much decaying vegetable matter (Fig. 50). [Illustration: FIG. 50.--Birds'-nest fungus (_Cyathus_). _A_, young. _B_, full grown. _C_, section through _B_, showing the "sporangia" (_sp._). All twice the natural size.] In the second order the spores are at first protected, as we have seen in _Coprinus_, which belongs to this order, but finally become exposed. Here belong the toadstools and mushrooms (Fig. 51, _B_), the large shelf-shaped fungi (_Polyporus_), so common on tree trunks and rotten logs (Fig. 51, _C_, _D_, _E_), and the prickly fungus (_Hydnum_) (Fig. 51, _G_). [Illustration: FIG. 51.--Forms of _Basidiomycetes_. _A_, _Tremella_, one-half natural size. _B_, _Agaricus_, natural size. _C_, _E_, _Polyporus_: _C_, × ½; _E_, × ¼. _D_, part of the under surface of _D_, natural size. _F_, _Clavaria_, a small piece, natural size. _G_, _Hydnum_, a piece of the natural size.] Of the last, or naked-fruited forms, the commonest belong to the genus _Clavaria_ (Fig. 51, _F_), smooth-branching forms, usually of a brownish color, bearing the spores directly upon the surface of the branches. CHAPTER XI. SUB-KINGDOM IV. BRYOPHYTA. The Bryophytes, or mosses, are for the most part land plants, though a few are aquatic, and with very few exceptions are richly supplied with chlorophyll. They are for the most part small plants, few of them being over a few centimetres in height; but, nevertheless, compared with the plants that we have heretofore studied, quite complex in their structure. The lowest members of the group are flattened, creeping plants, or a few of them floating aquatics, without distinct stem and leaves; but the higher ones have a pretty well-developed central axis or stem, with simple leaves attached. There are two classes--I. Liverworts (_Hepaticæ_), and II. Mosses (_Musci_). CLASS I.--THE LIVERWORTS. One of the commonest of this class, and to be had at any time, is named _Madotheca_. It is one of the highest of the class, having distinct stem and leaves. It grows most commonly on the shady side of tree trunks, being most luxuriant near the ground, where the supply of moisture is most constant. It also occurs on stones and rocks in moist places. It closely resembles a true moss in general appearance, and from the scale-like arrangement of its leaves is sometimes called "scale moss."