PLANTING 
85 
potatoes were planted year after year in the same corner of 
a garden, the land would very likely become infected, and in 
time scarcely any potatoes in a whole harvest would be free 
from disease. If, on the other hand, when the first trace of 
scab appears, the potato patch is transferred to another spot, 
the fungus, faithful to its choice, is starved out. 
Insects, to be sure, allow themselves a larger range of food 
supply than fungi do, not remaining constant to one plant. 
But still the plan of shifting a group of plants from one part of 
a garden to another is, for the reasons already given, strongly 
advised. The hard-pushed gardener grimly enjoys giving 
young insects whose birthplace has been nicely selected by 
the mother the surprise of a lifetime in a total change of crop. 
Anybody who lives near a truck farm hears technical ex- 
pressions with which he becomes familiar. Gardeners talk, 
for example, about catch crops, cover crops, and green manure. 
By catch crop they mean a crop that is planted between two 
money-making crops. A cover crop means some crop planted 
late in the season, chiefly for the purpose of holding the sol- 
uble food which would otherwise drain away. Clover is per- 
haps the best, but winter wheat and rye and turnips also 
make good cover crops. These are usually plowed under in 
the spring ; they act in this way as a form of green manure. 
Green manuring means the planting of certain herbaceous 
plants for the sole purpose of enriching the soil. Some plant 
organisms are constituted so that they can successfully play 
this role of benefaetor to the land. Those that stand pre- 
eminent as great soil renovators are the leguminous plants. 
It is a fact that three representatives of this family, the clover 
in the north, and the cowpea and the alfalfa in the south, have 
rejuvenated miles of worn-out farm land. 
These few hints will at least serve to show how certain defi- 
nite changes in crops are planned by the farmer according 
