9 2 



PHYSIOLOGY. 



threads do assimilate from the humus certain unoxidized, or feebly oxi- 

 dized, nitrogenous substances (ammonia, for example), and transfer them 

 over to the host, for the higher plants with difficulty absorb these sub- 

 stances, while they readily absorb nitrates which are not abundant in 

 humus. This is especially important in the forest. It is likely therefore 

 that the fungus symbiont supplies nitrogen to its host, though it does not 

 assimilate free nitrogen as is the case in the following examples. 



5. Nitrogen gatherers. 



195. How clovers, peas, and other legumes gather nitrogen. It has long 

 been known that clover plants, peas, beans, and 

 many other leguminous plants are often able to 

 thrive in soil where the cereals do but poorly. 

 Soil poor in nitrogenous plant food becomes richer 

 in this substance where clovers, peas, etc., are 

 grown, and they are often planted for the purpose 

 of enriching the soil. Leguminous plants, espe- 

 cially in poor soil, are almost certain to have en- 

 largements, in the form of nodules, or "root 

 tubercles." A root of the common vetch with 

 some of these root tubercles is shown in fig. 81. 



196. A fungal or bacterial organism in these 

 root tubercles. If we cut one of these root tuber- 

 cles open, and mount a small portion of the in- 

 terior in water for examination with the micro- 

 scope, we shall find small rod-shaped bodies, 

 some of which resemble bacteria, while others are more or less forked into 

 forms like the letter Y, as shown in fig. 82. These bodies are rich in 

 nitrogenous substances, or proteids. They are portions of a minute organism, 

 of a fungus or bacterial nature, which attacks the roots of leguminous plants 



Fig. 81. 



Root of the common vetch, 

 showing root tubercles. 



Fig. 82. Fig. 83. 



Root-tubercle organism from vetch, old con- Root-tubercle organism from Medicago 

 dition. denticulata. 



and causes these nodular outgrowths. The organism (Phytomyxa legumi- 

 nosarum) exists in the soil and is widely distributed where legumes grow. 



