1 62 PLANT LIFE. 



fluid on the surface or in the interior of the organic material 

 upon which they flourish. Saprophytic fungi either form 

 their mycelium upon the surface of the organic matter, or, 

 more commonly, they penetrate it more or less extensively 

 by a profusely branched system of submerged hyphse. A few 

 saprophytic seed plants form at the base of the stem an en- 

 larged, tuber-like mass from whose surface great numbers of 

 profusely branched roots arise. These penetrate the decay- 

 ing material in all directions, and act as absorbing organs. 

 A few have abundantly branched underground stems and 

 have no permanent roots. 



219. 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 foods with which they are in contact. 

 This is brought about either by a direct action of the proto- 

 plasm of the living plant, or by means of enzymes (^[ 237) 

 excreted by it. Such chemical changes, by means of which 

 insoluble solid materials are transformed into soluble ones 

 and are dissolved, are quite like those which occur in the di- 

 gestive tract of the higher animals, and, therefore, may be 

 properly termed digestion. 



220. 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.* 



221. Fermentation and putrefaction. — Some saprophytes 

 produce changes in the material upon or in which they grow, 

 other than those described above. The more important 

 changes may be comprehended under the two terms fermen- 

 tation and putrefaction. Between these there is no sharp 



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

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

 writers. 



