246 A TEXTBOOK OP BOTANY [Ch. V, 6 



however, and conspicuously the Pulse family, the relation is 

 /more direct, for the nitrogen-fixing Bacteria live in the 

 / tissues, in the nodules so familiar on the roots of Beans 

 ' and Peas (Fig. 173), to which the compounds are thus sup- 

 plied with minimal loss. There is obvious connection be- 

 tween this economical arrangement and the fact that the 

 seeds of Leguminosae are richest of all plant products in 

 nitrogenous substances, particularly proteins, thus coming 

 j nearest to meat in food value. 



The importance of nitrogen-fixing Bacteria in soil fertility 

 has of course suggested the attempt to enrich poor soils by 

 adding the suitable Bacteria thereto. Many attempts have 

 been made to this end, but while successful as laboratory 

 experiments, they have not as yet achieved importance in 

 practice. 



To complete the subject of nitrogen acquisition by the 

 higher plants, we should note that such has been held to 

 explain the insectivorous habits of the pitcher plants and 

 others which trap insects (page 76). The plants which 

 capture insects digest the bodies thereof, and absorb into 

 their own tissues the resultant substances, which of course 

 are particularly rich in nitrogenous materials. In general, 

 the insectivorous plants are found in places where the 

 nitrifying Bacteria of soils are unlikely to be found, — our 

 Sarracenias and Sundews in bogs, the Venus Fly-trap in 

 sand, and the Nepenthes on the trunks of trees. 



Protozoa are minute one-celled animals, typified by the 

 creeping Amoeba. They abound in rich soils, the fertility 

 of which they are now claimed to influence. It is found that 

 any methods of treatment, by heat or poisons, which kill 

 these Protozoa but not the Bacteria, produce increased 

 fertility; and since it is likely the Protozoa feed upon 

 Bacteria, the inference is drawn that the destruction of the 

 former permits increase in numbers of the latter, with pro- 

 portionally better nitrification and nitrogen-fixation. Here 

 again, however, we must await further evidence. 



