The haustoria of the dodder form a complete junction with the vascular bundles of its host so that through the vessels water and salts are obtained, while through the junction of sieve tubes the elaborated organic food is obtained. The union of the dodder with its host is like that between a graft and the graft stock. The beech drops (Epiphegus) is another example of a parasitic seed plant. It is parasitic on the roots of the beech. [Illustration: Fig. 74. Dodder.] =180. The mistletoe= (Viscum album), which grows on the branches of trees, sends its roots into the branches, and only the vessels of the vascular system are fused according to some. If this is true then it probably obtains only water and salts from its host. But the mistletoe has green leaves and is thus able to assimilate carbon dioxide and manufacture its own organic substances. It is claimed by some, however, that the host derives some food from the parasite during the winter when the host has shed its leaves, and if this is true it would seem that organic food could also be derived during the summer from the host by the mistletoe. =181. Saprophytes.=—A saprophyte is a plant which is enabled to obtain its food, especially its organic food, directly from dead animals or plants or from dead organic substances. Many fungi are saprophytes, as the moulds, mushrooms, etc. (See Nutrition of the Fungi.) =182. Humus saprophytes.=—The action of fungi as described in the preceding chapter, as well as of certain bacteria, gradually converts the dead plants or plant parts into the finely powdered brown substance known as _humus_. In general the green plants cannot absorb organic food from humus directly. But plants which are devoid of chlorophyll can live saprophytically on this humus. They are known as _humus saprophytes_. Many of the mushrooms and other fungi, as well as some seed plants which lack chlorophyll or possess only a small quantity, are able to absorb all their organic food from humus. It is uncertain whether any seed plants can obtain all of their organic food directly from humus, though it is believed that many can so obtain a portion of it. But a number of seed plants, like the Indian-pipe (Monotropa) and certain orchids, obtain organic food from humus. These plants lack chlorophyll and cannot therefore manufacture their own carbohydrate food. Not being parasitic on plants which can, as in the case of the dodder and beech drops mentioned above, they undoubtedly derive their organic food from the humus. But fungus mycelium growing in the humus is attached to their roots, and in some orchids enters the roots and forms a nutritive connection. The fungus mycelium can absorb organic food from the humus and in some cases at least can transfer it over to the roots of the higher plant (see Mycorhiza).