AND SAPROPHYTES. 85 



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 pre- 

 ceding 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 be- 

 lieved 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 1 . 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). 



183. Autotrophic, heterotrophic, and mixotrophic plants. An auto- 

 trophic 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 



