NUTRITION. 137 



their natural form completely or not, are known as saprophytes. 

 Those organisms which live in association one with another 

 are called symbionts and their relation is known as symbiosis. 

 (See Chap. XXIV.) If one plant preys upon and injures 

 another living plant or animal, it is called 2^ parasite and the 

 being which supports it is called its host. (See Tf 44. ) 



181. Saprophytes. — Saprophytic bacteria live immersed 

 in solutions of food, or surrounded by films of fluid on the 

 surface or in the interior of the solid material upon which they 

 flourish. ■ Saprophytic fungi either form their mycelium upon 

 the surface of the substratum, which contains their food, or, more 

 commonly, they penetrate it more or less extensively by a pro- 

 fusely branched system of hyphae. A few saprophytic seed 

 plants form at the base of the stem an enlarged, tuber-like mass 

 from whose surface great numbers of profusely branched roots 

 arise. These penetrate the decaying food material in all direc- 

 tions, and act as absorbing organs. A few have abundantly 

 branched underground stems and have no permanent roots. 



182. Digestion, — Saprophytes whose surfaces are sur- 

 rounded by food solutions have only to absorb them. Some, 

 however, have power to convert ""into material soluble in 

 water the solid insoluble food with which they are in contact. 

 This is brought about in most cases by substances excreted by 

 the living protoplasm. Such chemical changes, by means of 

 which insoluble solid materials are transformed into soluble 

 ones and are dissolved, are identical in nature with those 

 which occur in the digestive tract of the higher animals, and, 

 therefore, may be properly termed digestion. 



183. Assimilation. — After the food is absorbed, it under- 

 goes various changes, collectively known as assimilation, by 

 which it is enabled to become part of the living material of 

 the plant body.* 



* This is not to be confused with the manufacture of food by green 

 plants, to which the term assimilation is inaptly applied by most writers. 



