SUMMARY 201 



with cultivated annuals. These developments, called "bac- 

 teriorrhiza," have been noted in the epidermis, cortex, or even the 

 bast, both in the interior of the cells and in the intercellular spaces. 

 Whether they are of general or exceptional occurrence, whether 

 parasitic to the plants or favorable to their development, is still 

 undetermined. 



Just as different fungi may penetrate plant roots to a greater 

 or less extent, causing definite injury or exerting favorable effects 

 on plant development, so bacteria are also not excluded entirely 

 from entrance into the roots. It has been shown in the preceding 

 pages that bacteria make very extensive development on the 

 immediate root surfaces. Some cells of bacteria penetrate the 

 tissues to greater depths, acting merely as saprophytes upon the 

 degenerating or weak tissues. Some pathogenic organisms pro- 

 duce much more extensive penetration and cause pronounced 

 injurious effects. 



Summary. — Microorganisms may thus be considered to affect 

 root systems of higher plants in numerous ways. At some dis- 

 tance from the roots, the microorganisms may either (1) act as 

 general agents of decomposition of organic constituents of the soil 

 leading to the formation of water, carbon dioxide, ammonia, sul- 

 fates, and phosphates; (2) act as transformers of such mineral 

 constituents of the soil as ammonia and sulfur, oxidizing them to 

 nitrate and sulfate, respectively; (3) act as agents of assimilation 

 of nutrients which are thus removed at least temporarily from the 

 zone of absorption of plants; (4) act as agents lowering the oxygen 

 concentration in the soil system and thus create conditions unfav- 

 orable to root growth; (5) produce toxic substances or reduce such 

 substances as nitrates and sulfates to gaseous nitrogen and sul- 

 fides, thus rendering them unavailable; (6) act as solvent agents 

 through the organic and inorganic acids which are produced in 

 various transformations; (7) act as nitrogen-fixing organisms, the 

 nitrogen sooner or later becoming available to higher plants. 



Close to the root systems many similar effects are exerted, but 

 the changes take place more rapidly because of the accelerated 

 activity of microbes in the rhizosphere. These influences may 

 include: (1) decomposition of organic products of excretion from 

 the roots; (2) solvent action of the carbon dioxide which arises 

 both from microbial activity and root excretion; (3) fixation of 

 nitrogen where the organic root excretions contain little or no 



