84 
GARDENS AND THEIR MEANING 
land, after a few years, got " all wore out.” The plan of regu- 
larly changing crops is in a sense new, and yet on the best 
farms rotation has long been in vogue even when the scientific 
reasons underlying the practice have not been fully understood. 
The method, after all, is nature’s own. Whatever may be 
the explanation, nobody can have failed to observe how uni- 
versally a natural rotation takes place in the yield of wild 
land. Let an oak grove be doomed to the ax, and lo ! up 
springs a pine thicket. Cutting off the pines in their turn 
gives a signal for young birches to step quietly in. As for 
maple and ash clearings, the owner can scarcely turn around 
before the tangles of low-bush blueberry are up knee-high. 
Not only is the amount of nutriment in land a matter which 
a farmer must understand, but it is necessary for him to know 
how deep the roots of a plant will strike to get its food. In 
this respect plants vary surprisingly. Clover and alfalfa roots 
are able to penetrate several feet ; sugar beets and parsnips 
will not push down so far, but they will always root deeper 
than table beets and onions. Therefore in order to extract the 
food materials economically, — and this means more or less 
evenly, — it will be advisable in rotating to choose plants that 
feed at different depths. It is not uncommon for a farmer 
to use certain deep-rooting plants, like the turnips, to bring to 
the surface of the land food materials that lie out of the reach 
of his ordinary crop. 
There is another reason for moving a given crop from place 
to place. Every crop brings in its wake peculiar troubles of 
its own. One set of grievances to the farmer and his crops 
arises from fungous diseases; another comes from insect pests. 
The spores, or seedlike bodies, of each fungus thrive upon a 
particular plant and almost exclusively upon that one plant. 
Take the spore of the potato scab, for instance ; this will 
grow on potatoes, but, as a rule, on no other vegetable. If 
