498 BIOLOGY AND HUMAN LIFE 



the surface and may be seriously delayed. If the plants are too 

 far apart, there will be more work per thousand plants or less 

 yield per acre ; if they are too close together, they will crowd 

 each other. By means of rotation of crops the nitrogen re- 

 moved by some crops is restored by the nitrogen-fixing bacteria 

 that cooperate with the plants of the bean family. Then there 

 is the weeding, which must be done in a way that will produce 

 the most favorable results for the crop. 



4. Protection. Cultivated plants, in order to yield both the 

 best quality and the largest quantity, must be protected from 

 their many dangers and enemies. The temperature has to be 

 considered, as in deciding upon the best time for planting. 

 Sometimes it is necessary to have seedlings ready to set out 

 when the frost is gone ; sometimes it is necessary to shield plants 

 from excessive sunshine ; and often the wind is a serious factor, 

 especially in grain-raising and fruit-raising. The most serious 

 problem in crop-raising of all kinds is the blighting effect of 

 various plants and animals — chiefly various species of fungi 

 and of insects (see Chapter XLIX). The protection consists of 

 a great variety of measures : tfie selection of seeds and tubers 

 that are free from infection ; the sterilizing of seeds and tubers ; 

 the use of powders and sprays on the growing plants. For each 

 particular kind of crop and for each particular kind of enemy a 

 definite procedure must be worked out. The many stunted, 

 blotchy, speckled, or twisted potatoes and other vegetables and 

 fruits that come to market give only a slight suggestion of the 

 great extent to which our useful cultivated plants are injured 

 by these enemies. 



5. Making the most of our animals. The management of ani- 

 mals, like the management of plants, depends upon an increasing 

 knowledge of the factors that make for their healthy growth 

 and development, and that protect them from dangers and 

 disease. There is of course the question of suitable food — suit- 

 able in quality and in proportions, or balance, as well as in total 

 quantity; and it must meet the needs of different ages. If a 

 calf has been underfed, he will have a comparatively large head, 



