=186. Nutrition of parasitic fungi.=—Certain of the fungi grow on or within the higher plants and derive their food materials from them and at their expense. Such a fungus is called a _parasite_, and there are a large number of these plants which are known as _parasitic fungi_. The plant at whose expense they grow is called the “_host_.” One of these parasitic fungi, which it is quite easy to obtain in greenhouses or conservatories during the autumn and winter, is the carnation rust (_Uromyces caryophyllinus_), since it breaks out in rusty dark brown patches on the leaves and stems of the carnation (see fig. 75). If we make thin cross-sections through one of these spots on a leaf, and place them for a few minutes in a solution of chloral hydrate, portions of the tissues of the leaf will be dissolved. After a few minutes we wash the sections in water on a glass slip, and stain them with a solution of eosin. If the sections were carefully made, and thin, the threads of the mycelium will be seen coursing between the cells of the leaf as slender threads. Here and there will be seen short branches of these threads which penetrate the cell wall of the host and project into the interior of the cell in the form of an irregular knob. Such a branch is a _haustorium_. By means of this haustorium, which is here only a short branch of the mycelium, nutritive substances are taken by the fungus from the protoplasm or cell-sap of the carnation. From here it passes to the threads of the mycelium. These in turn supply food material for the development of the dark brown gonidia, which we see form the dark-looking powder on the spots. Many other fungi form haustoria, which take up nutrient matters in the way described for the carnation rust. In the case of other parasitic fungi the threads of the mycelium themselves penetrate the cells of the host, while in still others the mycelium courses only between the cells of the host (fungus of peach leaf curl for example) and derives food materials from the protoplasm or cell-sap of the host by the process of osmosis. [Illustration: Fig. 76. Several teleutospores, showing the variations in form.] [Illustration: Fig. 77. Cells from the stem of a rusted carnation, showing the intercellular mycelium and haustoria. Object magnified 30 times more than the scale.] [Illustration: Fig. 78. Cell from carnation leaf, showing haustorium of rust mycelium grasping the nucleus of the host. _h_, haustorium; _n_, nucleus of host.] [Illustration: Fig. 79. Intercellular mycelium with haustoria entering the cells. _A_, of Cystopus candidus (white rust); _B_, of Peronospora calotheca. (De Bary.)]