=190. How fungi derive carbohydrate food.=—The fungi being devoid of chlorophyll cannot assimilate the CO₂ from the air. They are therefore dependent on the green plants for their carbohydrate food. Among the saprophytes, the leaf and wood destroying fungi excrete certain substances (known as _enzymes_) which dissolve the carbohydrates and certain other organic compounds in the woody or leafy substratum in which they grow. They thus produce a sort of extracellular digestion of carbohydrates, converting them into a soluble form which can be absorbed by the mycelium. The parasitic fungi also obtain their carbohydrates and other organic food from the host. The mycelium of certain parasitic, and of wood destroying fungi, excretes enzymes (_cytase_) which dissolve minute perforations in the cell walls of the host and thus aid the hypha during its boring action in penetrating cell walls. NOTE.—Certain wood destroying fungi growing in oaks absorb tannin directly, i.e. in an unchanged form. One of the pine destroying fungi (_Trametes pini_) absorbs the xylogen from the wood cells, leaving the pure cellulose in which the xylogen was filtrated; while _Polyporus mollis_ absorbs the cellulose, leaving behind only the wood element. 4. Mycorhiza. =191.= While such plants as the Indian-pipe (Monotropa), some of the orchids, etc., are _humus saprophytes_ and some of them are possibly able to absorb organic food from the humus, many of them have fungus mycelium in close connection with their roots, and these fungus threads aid in the absorption of organic food. The roots of plants which have fungus mycelium intimately associated in connection with the process of nutrition, are termed _mycorhiza_. There is a mutual interchange of food between the fungus and the host, a _reciprocal symbiosis_. =192. Mycorhiza are of two kinds= as regards the relation of the fungus to the root; _ectotrophic_ (or _epiphytic_), where the mycelium is chiefly on the outside of the root, and _endotrophic_ (or _endophytic_) where the mycelium is chiefly within the tissue of the root. =193. Ectotrophic mycorhiza.=—Ectotrophic mycorhiza occur on the roots of the oak, beech, hornbean, etc., in forests where there is a great deal of humus from decaying leaves and other vegetation. The young growing roots of these trees become closely covered with a thick felt of the mycelium, so that no root hairs can develop. The terminal roots also branch profusely and are considerably thickened. The fungus serves here as the absorbent organ for the tree. It also acts on the humus, converting some of it into available plant food and transferring it over to the tree.