Parasitic fungi.—Most of the moulds are saprophytes. Many other fungi are parasitic on living plants and animals (Fig. [285]). Some of them have complicated life histories, undergoing many changes before the original spore is again produced. The willow mildew and the common rust of wheat will serve to illustrate the habits of parasitic fungi.
The willow mildew (Uncinula salicis).—This is one of the sac fungi. It forms white downy patches on the leaves of willows (Fig. [276]). These patches consist of numerous interwoven threads that may be recognized under the microscope as the mycelium of the fungus. The mycelium in this case lives on the surface of the leaf and nourishes itself by sending short branches into the cells of the leaf to absorb food materials from them.
Fig. 276.—Colonies of Willow Mildew.
Fig. 277.—Summer-spores of Willow Mildew.
Fig. 278.—Perithecium of Willow Mildew.
Fig. 279.—Section through Perithecium of Willow Mildew.