NUTRITION 379 



the free N 2 from the air in their bodies, which, dying, restore it to 

 the soil. 



Leguminosae. — The case of the Leguminosae and a few other plants 

 is peculiar. Certain soil bacteria enter the young root hairs, grow 

 and multiply, and work gradually into the cortex, where, as they increase, 

 they stimulate the rootlet to multiply and enlarge the cortical cells, so 

 that a local swelling or tubercle is formed. The largest of these scarcely 

 exceeds the size of a hazelnut, and most are smaller than a pea or even 

 a grain of wheat. The relations are probably as follows : The bacteria 

 depend on their host for carbohydrate food, but can use the free nitrogen 

 (presumably that nearest them in solution, which is replaced from the 

 air) in their protein making. Being favorably situated, many of the bac- 

 teria become excessively enlarged, and often branch into X and Y forms. 



The host sooner or later gets the better of the parasite and consumes 

 these fat bacteria (" bacteroids "), their proteins proving valuable foods. 

 By reason of this peculiar relation, leguminous crops can be grown in 

 soils which contain no combined nitrogen whatever, provided the proper 

 bacteria be present. 1 If the crop be then plowed under (a process 

 called green manuring), the soil is enriched in nitrogen at the expense 

 of the air. 2 (See further Part III on root tubercles.) 



Source of S and P. —The sulfur and phosphorus needed are obtained 

 by the green plants from sulfates and phosphates which dissolve in the 

 soil water. Few soils lack these, though for cropping the phosphates 

 may be insufficient or may be so reduced as to interfere with full devel- 

 opment. " Land plaster " (gypsum, or calcium sulfate) is sometimes 

 applied to fields; but it probably has more beneficial effects on other 

 qualities than on the composition of the soil. Phosphates are an impor- 

 tant part of artificial manures. 2 In the case of both nitrogen and phos- 

 phorus it is highly important, if immediate effects are desired, that the 

 compounds be such as are " available," and compounds can be available 

 only when they are soluble or readily become so. 



Raw materials. — The nitrates, sulfates, and phosphates enter the 

 larger plants through the roots. These are the mineral salts which are 

 most necessary for the well-being of the plant, because they are needed for 



1 If not, the soil may be infected by scattering on it soil in which such a crop has been 

 previously grown. Commercial attempts to supply pure cultures of appropriate bacteria 

 for infecting the soil through the seed sown have not been very successful. 



2 The whole subject of the relation of manures and fertilizers to the soil and crop is in 

 a very unsatisfactory state and needs further investigation before the practice and results 

 can be explained. 



