166 THE PRINCIPLES OF AGRICULTURE 



orous from the beginning, rather than those 

 which are forced into very heavy growth after a 

 long period of neglect. 



288. The heading- in of young growths tends 

 to force out the side shoots and to develop the 

 dormant buds. The more a plant is headed- in, 

 therefore, the more thinning- out it will require. 

 Heading-iii induces fruitfulness by checking 

 growth and by encouraging the formation of 

 side spurs (upon which fruit may be borne) . 



289. Heavy pruning every few years which 

 is the custom tends to keep trees over-vigorous 

 and unproductive. Mild pruning every year 

 maintains the equilibrium of the plant, and tends 

 to make it fruitful. 



3. By Keeping Enemies in Check 

 3a. The kinds of enemies 



290. Of plant enemies or diseases, there are 

 three main types, insects, parasitic fungi, con- 

 stitutional or physiological troubles. 



291. Insect pests are of two general types, so 

 far as their method of feeding is concerned, 

 insects which chew, or bite off pieces of the plant, 

 and those which suck their food from the juices 

 of the plant. In the former class are the worms 

 and beetles ; in the latter are plant -lice, scale 

 insects, and the so-called true bugs (as the 



