76 PLANT PHYSIOLOGY 



The saprophytic bacteria make up what are called the " bac- 

 teria of decay" and, although they may cause vile odors and tastes, 

 they are, on the whole, beneficial to man. They cause the return 

 of organic matter to the inorganic state, where it can again be 

 used. If it were not for these agents of decay, when an organism 

 died the complex molecules of organic matter would remain in a 

 condition where they would be unavailable for the use of other 

 organisms. The dead plants and animals would long ago have 

 smothered out life on the surface of the earth, and the world would 

 in a peculiar and fitting sense be "dead." The saprophytic bac- 

 teria and allied forms, however, are able to continue the organic 

 cycle and to derive their energy in the process. 



The Symbionts. — The symbionts may similarly be divided 

 into two groups: those which live mutualistically and those which 

 are parasitic. Among the important symbionts are the my cor- 

 rhiza which consist of a web of fungal filaments that surround the 

 roots of a plant and aid it in obtaining mineral nutrients from the 

 soil. These have been found to be of two types. In the endo- 

 trophic mycorrhiza as found in many of the halophytes (Plantago 

 coronopus, Glyceria maritima), in many heaths (Andromeda), 

 in black maple (Acer nigrum), horse-chestnut (^Esculus), and 

 walnut (Juglans), the fungal filaments penetrate the cortex of 

 the root and enter the cells curling up inside. In the ectotrophic 

 mycorrhiza of the birch (Betula), hickory (Carya), beech (Fagus), 

 oak (Quercus), and sugar maple (A. saccharum) the hyphse cover 

 only the surface of the roots and do not penetrate into the interior. 

 Mycorrhiza are of very common occurrence, and forest trees 

 growing in rich humus seem generally to be associated with such 

 fungi. 



The fungus obtains organic food from the tree and in turn gives 

 it water and mineral salts. The soil, under the conditions found 

 on the forest floor, is a seething mass of bacteria, higher fungi, 

 and other microorganisms. This living environment of the roots 

 makes the competition for mineral salts especially keen, since 

 all the organisms present in the soil need a certain amount of the 

 available salts. It thus appears that plants with mycorrhiza as- 

 sociated with them have a much better chance to compete with 

 the soil organisms than those which contain no mycorrhiza. In 

 this way both the tree and the fungus seem to be benefited, al- 

 though some investigators think that the relation is much to the 



