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 po 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. 
