l6o PHARMACEUTICAL BACTERIOLOGY 



for enriching the soil. Incidentally, it is of interest to know that Virgil's 

 agricultural epic just referred to was primarily intended as a strong plea 

 for turning the minds of the idle rich back to the soil, a plea which was 

 never more urgent than it is at the present time. 



The Roman farmers practiced crop rotation, summer fallowing, green 

 manureing, and considered the bean, the vetch and luzerne (alfalfa) as 

 special enrichers of the soil. The farmers of middle Europe early acquired 

 a knowledge of these cultural operations from their Roman neighbors. 

 Red clover soon became known as a valuable enricher of the soil, and has 

 been long used for that purpose by the farmers of France, Germany and 

 England. The Chinese and Hindoos have for thousands of years made 

 use of a pressed bean fertilizer in rice culture. Transfer of a rich soil top 

 dressing to new or arid fields has been practiced for many centuries. In 

 those remote times the value of soil tillage, of soil warmth, of fertilizers, 

 was fully recognized but it is only within recent years that the true signifi- 

 cance of these basic agricultural factors has been discovered. 



Plant Growth and Bacteria. The bacteria concerned in plant growth 

 may conveniently be divided into three great groups, as follows: 



1. Those that are a part of the soil. That is, those bacteria which are 

 normally present in the soil and which feed upon the organic matter 

 (humus) in the soil, rendering the humus available for the use of plants 

 which may be growing in the soils. 



2. Bacteria which live upon and in the immediate vicinity of the roots 

 of plants. These bacteria are essential to the normal development of 

 plants and each kind of plant has its own special kind or kinds of bacterial 

 associates. The bacteria and the host plants form a mutualistic associa- 

 tion, that is an association for mutual gain and benefit (mutualism, or 

 mutualistic symbiosis). 



Organisms other than bacteria may form such beneficient associations 

 with higher plants. Thus, we find molds in mutualistic association upon 

 the terminal rootlets of oak seedlings and upon the roots of other repre- 

 sentatives of the oak family. Some of the soil algae, under certain condi- 

 tions, will enter into beneficient relationships with certain plants (mints, 

 calamus, iris, sedges, and other wet soil plants). An alga (Nostoc) forms 

 a mutualistic association with the cycad (Cycas rewluta). 



3. Bacteria which live within the root tissues of plants. Perhaps every 

 farmer and gardener has observed certain nodules on the roots and rootlets 

 of plants belonging to the bean family, as bean, pea, lentil, soy bean, 

 alfalfa, clover, peanut, cassia, lupine, melilotus, etc. The internal tissue 

 cells of these nodules contain billions of bacteria which have the power of 

 binding the free nitrogen of the air, converting it into nitrogenous com- 

 pounds which are utilized by the host plants and which upon the death of 



