NUTRITION 93 
exhaust or use up the mineral "nutrients" more rapidly 
than a succession of the different kinds of crops, nor does 
it introduce into the soil any permanent excrement toxic 
to wheat. It does, however, tend to introduce with the 
seed, stubble, roots, et cetera a number of kinds (at least 
five) of parasitic fungi that cause diseases of the wheat 
plants. These fungi destroy, blight, and rot off the roots 
of the plants, and live internally in the straw and the 
seeds. The accumulation of these fungi in fertile soils 
brings about the condition of wheat-sickness, "wheat- 
tired" soil. The fungi attack the roots, leaves, stems, 
young developing grains, and seedlings, and the value of 
crop-rotation lies in growing a series of different kinds of 
crops that do not transmit or bear each other's diseases. 
Crop-rotation is not primarily to conserve the fertility of 
the soil, but is a sanitary measure, tending to eradicate 
contagious disease. The reproductive bodies (spores) of 
these fungi are carried from field to field and persist in the 
field for some time, but lose their vitality during the few 
seasons when other crops are being cultivated. It is thus 
seen that one farmer, by careless methods of agriculture, 
may not only suffer a loss of yield of his own crops, but 
may also infect his neighbors' fields. In addition to crop- 
rotation, this trouble may be reduced or avoided by care- 
fully sterilizing all seed, before sowing, by soaking them 
in a weak solution of formaldehyde, and also by sterilizing 
the soil in a similar manner. The validity of this theory 
is based upon extended studies of wheat, oats, barley, 
and flax: it doubtless holds true also for other crops. 
These three theories are all based upon thorough experi- 
mental investigation, and it is probable that all three 
contribute to a rational basis for the rotation of crops. 
