244 NUTRITIVE METABOLISM 



membranes of absorbing organs. It is definitely established how- 

 ever that all green plants take up minute proportions of such 

 organic food, and to that extent lessen their need for the products 

 of photosynthesis. A large number of plants including the great 

 groups of bacteria (except a few forms, 303, 304) and fungi have 

 lost the power of forming chlorophyl, and of photosynthesis, and 

 obtain their food from substances absorbed from living or dead 

 organisms. This variation is shared to some extent by seed 

 plants also. Species which derive their food from decaying organic 

 matter are termed saprophytes. 1 Only one species of seed plant 

 is supposed to be able to live wholly in this manner, although this 

 point needs further investigation. A large number exhibit various 

 degrees of saprophytism however, among which are to be re- 

 counted the carnivorous forms which receive or entrap animals, the 

 decaying remains of which are used by the plant. 



Many species attach themselves to the bodies of other organ- 

 isms, and derive all of their food-supply from their host, or only a 

 part of it, being furnished with a modicum of chlorophyl, and 

 hence able to carry on some photosynthesis. 



In another general type of nutrition two or more species as- 

 sociate together in such manner that an exchange of material en- 

 sues between them, resulting in various degress of benefit to the 

 members of the partnership. Such associations constitute a sym- 

 biosis. One form of such symbiosis, in which saprophytic fungi 

 are associated wijth the underground organs of various pterido- 

 phytes, gymnosperms and phanerogams, constituting mycorhizas, 

 is very widely prevalent. Perhaps the greater number of all the 

 higher plants enter into such combinations, and receive a small 

 proportion of their total food-supply by exchange with the fungi 

 attached to their underground organs. Certain fungi and algae 

 associate in this manner to form the lichens, a distinct group in 

 the vegetable kingdom. 



313, Nutrition of a Saprophyte. The following test will demon- 

 strate the substances used by a saprophytic fungus. Soak a slice 



1 MacDougal,. Symbiosis and saprophytism. Cont. N. Y. Botanical Garden. No. 

 I. 1899. (Rep. Bull. Torr. Bot. Club. 1899.) 



