92 



PHYSIOLOGY. 



threads (\o assimilate from tlie Iiiimus certain unoxidized, or feebly oxi- 

 dized, iiitroijeiioLis substances (amiiKjnia, U)V exainple), and transfer tliem 

 over to the liost, lor the hij^her [>hints with difficuU\' aljsorb these sub- 

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

 humus. This is especially important in the l<jrest. 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, aad other legumes gather nitrogen. — Jthas long 

 been known that clover plants, peas, beans, and 

 many other leguminous plants are often able to 

 thrive in soil where tlie cereals do but poorly. 

 Soil poor in nitrogenous plant f<x>d becomes richer 

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

 grown, an^.l 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. 8i. 



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 (tf the in- 

 terior in water for examinati(jn with the micro- 

 scope, we shall find small rod-shaped bodies, 

 some of which resemble bacteria, wliile otiiers are m<.re or less forked into 

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

 nitrogenous sul:)stances, or proleiils. TIie"\^ are portions of a minute organism, 

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



Fig. 81. 

 Root of the common vetch, 

 showing root tuberclss. 



^ m 



Fig 82 

 Root-tubercle organism trori 



Fig. 83. 

 Root-tiibcrcle organism from Medicago 



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

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



