Of the lower _Basidiomycetes_, the rusts (_Uredineæ_) offer common and easily procurable forms for study. They are exclusively parasitic in their habits, growing within the tissues of the higher land plants, which they often injure seriously. They receive their popular name from the reddish color of the masses of spores that, when ripe, burst through the epidermis of the host plant. Like many other fungi, the rusts have several kinds of spores, which are often produced on different hosts; thus one kind of wheat rust lives during part of its life within the leaves of the barberry, where it produces spores quite different from those upon the wheat; the cedar rust, in the same way, is found at one time attacking the leaves of the wild crab-apple and thorn. [Illustration: FIG. 47.--_A_, a branch of red cedar attacked by a rust (_Gymnosporangium_), causing a so-called "cedar apple," × ½. _B_, spores of the same, one beginning to germinate, × 300. _C_, a spore that has germinated, each cell producing a short, divided filament (basidium), which in turn gives rise to secondary spores (_sp._), × 300. _D_, part of the leaf of a hawthorn attacked by the cluster cup stage of the same fungus, upper side showing spermogonia, natural size. _E_, cluster cups (_Roestelia_) of the same fungus, natural size. _F_, tip of a leaf of the Indian turnip (_Arisæma_), bearing the cluster cup (_Æcidium_) stage of a rust, × 2. _G_, vertical section through a young cluster cup. _H_, similar section through a mature one, × 50. _I_, germinating spores of _H_, × 300. _J_, part of a corn leaf, with black rust, natural size. _K_, red rust spore of the wheat rust (_Puccinia graminis_), × 300. _L_, forms of black-rust spores: i, _Uromyces_; ii, _Puccinia_; iii, _Phragmidium_.] The first form met with in most rusts is sometimes called the "cluster-cup" stage, and in many species is the only stage known. In Figure 47, _F_, is shown a bit of the leaf of the Indian turnip (_Arisæma_) affected by one of these "cluster-cup" forms. To the naked eye, or when slightly magnified, the masses of spores appear as bright orange spots, mostly upon the lower surface. The affected leaves are more or less checked in their growth, and the upper surface shows lighter blotches, corresponding to the areas below that bear the cluster cups. These at first appear as little elevations of a yellowish color, and covered with the epidermis; but as the spores ripen they break through the epidermis, which is turned back around the opening, the whole forming a little cup filled with a bright orange red powder, composed of the loose masses of spores.