=183. Autotrophic, heterotrophic, and mixotrophic plants.=—An _autotrophic_ plant is one which is self-nourishing, i.e. it is provided with an abundant chlorophyll apparatus for carbon dioxide assimilation and with absorbing organs for obtaining water and salts. Heterotrophic plants are not provided with a chlorophyll apparatus sufficient to assimilate all the carbon dioxide necessary, so they nourish themselves by other means. _Mixotrophic_ plants are those which are intermediate between the other two, i.e. they have some chlorophyll but not enough to provide all the organic food necessary, so they obtain a portion of it by other means. Evidently there are all gradations of mixotrophic plants between the two other kinds (example, the mistletoe). =184. Symbiosis.=—Symbiosis means a living with or living together, and is said of those organisms which live so closely in connection with each other as to be influenced for better or worse, especially from a nutrition standpoint. _Conjunctive_ symbiosis has reference to those cases where there is a direct interchange of food material between the two organisms (lichens, mycorhiza, etc.). _Disjunctive_ symbiosis has reference to an inter-life relation without any fixed union between them (example, the relations between flowers and insects, ants and plants, and even in a broad sense the relation between saprophytic plants in reducing organic matter to a condition in which it may be used for food by the green plants, and these in turn provide organic matter for the saprophytes to feed upon, etc.). _Antagonistic_ symbiosis is shown in the relation of parasite to its host, _reciprocal_ symbiosis, or _mutualistic_ symbiosis is shown in those cases where both symbionts derive food as a result of the union (lichens, mycorhiza, etc.). 3. How Fungi Obtain their Food. [Illustration: Fig. 75. Carnation rust on leaf and flower stem. From photograph.] =185. Nutrition of moulds.=—In our study of mucor, as we have seen, the growing or vegetative part of the plant, the mycelium, lies within the substratum, which contains the food materials in solution, and the slender threads are thus bathed on all sides by them. The mycelium absorbs the watery solutions throughout the entire system of ramifications. When the upright fruiting threads are developed they derive the materials for their growth directly from the mycelium with which they are in connection. The moulds which grow on decaying fruit or on other organic matter derive their nutrient materials in the same way. The portion of the mould which we usually see on the surface of these substances is in general the fruiting part. The larger part of the mycelium lies hidden within the substratum.