64 PLANT GROWTH 



of about an inch or more should be painted with a lead and 

 oil paint to prevent infection by bacteria or fungi, which 

 might enter the open pores of the wood and cause the center 

 of the tree to rot. If limbs of two inches or more are to be 

 removed it may be difficult to avoid splitting into the tree. 

 It is best to cut first from the lower side of the branch about 

 one third through, about eighteen inches from where it is to 

 be removed, then about six inches farther out cut from above 

 until the limb breaks off. Now the stub can be removed 

 close to its base with no danger of splitting to injure the tree. 



Pruning stimulates the growth of buds below the cut as 

 has been suggested above. All terminal buds appear to 

 secrete a hormone which descends and inhibits the growth 

 of the buds which receive it, causing them to remain dor- 

 mant. When a twig is removed the first bud back of the cut 

 will cease to get the growth-inhibiting hormone and will 

 begin to grow. It is for this reason one can so easily deter- 

 mine the shape of a plant by judicious pruning. Simply 

 expect the topmost bud to grow in the direction it is pointed. 

 Trees may be trained to fill in sparse areas of their branching 

 by pruning a little more heavily on the strong side and 

 leaving buds which point toward the weak sides. 



Pruning appears to increase the vigor of a plant. This is 

 a balance of root and shoot problem, and therefore means 

 that the reduction of the top decreases the supply of carbo- 

 hydrates to the point where nitrogen is available in excess. 

 High nitrogen favors the growth of the protoplasm for larger, 

 thin-walled cells, resulting in more rapid growth. If more 

 new growth is desirable it is safe to try pruning more heavily. 

 It is clear from this and the preceding paragraphs that it is 

 unwise to prune a plant heavily in late summer because the 

 growth so stimulated would be tender and therefore in 

 danger of winter-killing. 



Tree surgery is rapidly becoming a respected profession 

 because more and more science is being applied. Tree "sur- 



