196 THE STUDY OF PLANT COMMUNITIES ■ Chapter VIII 



FlG. 92. Two seedlings of Psychotria punctata, about three and one-half 

 months old. The plant on the right is normal both as to growth and the pres- 

 ence of bacterial nodules dotting every leaf. The one on the left, grown bac- 

 teria-free, has reached its maximum development.— From Hwmn. 



127 



enter the root hairs of most legumes when available and produce 

 a proliferation of cortical cells sufficient to appear as a small 

 nodule on the root. Although the plant provides food for the bac- 

 teria and produces the nodule in which the bacteria multiply, the 

 relationship is truly symbiotic. These nitrogen-fixing bacteria are 

 able to take free nitrogen from the air, unavailable to most 

 plants, and to combine it with other elements to form compounds 

 that can be used by the plant during its lifetime. After death of the 

 plant, the accumulated nitrogenous compounds are released in the 

 soil and are used by other plants growing there. Legumes and 

 nitrogen-fixing bacteria are, therefore, important factors in main- 

 taining soil fertility in natural or cultivated soils. Plant commu- 

 nities becoming established on poor sites, such as eroded slopes, 

 invariably include a number of legumes, which are, of course, 

 particularly adapted to colonizing sterile or nitrate-depleted soils 

 and contributing to their improvement. Agricultural practice in- 

 cludes legumes in most crop rotations, and worn-out lands are 

 rebuilt by cropping with legumes of some sort. 



Nodules produced by bacteria are found on the roots of a few 

 plants in families other than Leguminosae, but they are not of the 

 same type. Nodules containing bacteria are also formed on leaves 

 of a number of tropical plants, mostly in the family Rubiaceae. 



