SYMBIOTIC NITROGEN FIXATION i6i 



phosphorus, and other essential elements in the soil; and (3) the 

 loose aerated surface tends to conserve the moisture of the lower 

 layers which can be drawn on by the plant, thus making more 

 nearly optimum moisture conditions. 



Moisture.— The root systems of plants vary greatly with the 

 moisture content of the soil. Gain found that legumes grown in 

 moist soil spread widely and were full of water, became covered 

 with root hairs, and presented a large surface of young tissues. In 

 the dry soil the roots were less spreading and the epidermis was 

 greatly thickened. 



In moist soil the tubercles of the peas were scattered all over 

 the roots, were five or six times as abundant as in the dry soil and 

 were about four times as large and ovoid in shape. In the dry 

 soil, however, no tubercles were produced on the superficial roots. 

 At a depth of about 20 centimeters some tubercles were found 

 of a hemispherical shape and much smaller than those grown 

 in moist soil. 



On beans about twenty times as many tubercles were found in 

 the moist soil, and microscopic examinations showed important 

 difFerences in the number of bacteria present and the structure of 

 the tubercles. Similar results were obtained with lupines and 

 other plants. This is what is to be expected, for when the root 

 system is not actively functioning the nodules are slowly de- 

 stroyed by the nodule-forming bacteria within and the saprophytic 

 bacteria without. The nitrogen fixed by the plant is proportional 

 to the number and size of the nodules. Hence, the gains made in 

 combined nitrogen are dependent upon the water applied to the 

 legume. This optimum will vary with different soils. Kalanta- 

 rov found in a loam soil that nodule bacteria require for their 

 growth a minimum moisture content of about 30 per cent, 

 whereas Prucha found in a sandy soil the optimum moisture con- 

 tent was from 20 to 40 per cent. Wilson found that an increase 

 in moisture content from 35 to 45 per cent more than doubled 

 the production of nodules, while with an increase of from 45 to 

 55 per cent it was nearly doubled. Therefore, water is necessary 

 for the normal functioning of the plant and bacteria, and it tends 



