426 FUNDAMENTALS OF FRUIT PRODUCTION 



serves than the shoots and spurs of thinned trees and their carbohydrate 

 contributions to the tree as a whole come later and may amount to less. 



Furthermore the thinned is more open than the headed tree. Its 

 leaves are better exposed to light and presumably they are for that reason 

 somewhat more effective manufacturing organs. The more common 

 formation of fruit buds in the better exposed parts of the tree is evidence 

 on this point. The rather general production of fewer and smaller leaves 

 on spurs in the interior shaded portions of compact headed trees, in 

 contrast to the larger and more numerous leaves on the spurs of open 

 thinned trees, is another fact pointing to material differences in the rate 

 of carbohydrate accumulation in their fruiting wood. 



Still another reason for the difference in response from heading back 

 and from thinning out lies in a disturbance of an equilibrium within the 

 branch itself induced by heading back. Each branch, as it grows, may 

 be regarded as a system in equilibrium, comparable to that in the plant 

 as a whole. That is, there is a balance between part and part. If a 

 portion of the branch is removed this balance is disturbed. Equilibrium 

 is reestablished by regeneration of the part pruned away. Apparently 

 little readjustment is necessary after thinning out, because the equihb- 

 rium of the remaining branches is not disturbed. The adjoining parts 

 will function more nearly as they would, had no pruning been done. 



The Places of Thinning and of Heading in Pruning Practice. — The 

 preceding discussion shows that no rules can.be laid down as to the relative 

 amomits of heading and of thinning that should be given trees of a certain 

 kind or of a certain age. Rather is it necessary to study carefully each 

 problem as it arises, to interpret and to apply the general principles that 

 have been pointed out. In a general way, however, it may be stated that 

 both the development of a more extensive fruiting system and more 

 especially the better and more efficient functioning of that system are 

 favored more by thinning than by heading. There are notable instances 

 of other effects, however, e.g. in the bramble fruits, in which the heading 

 back of the canes or other growth limits the energies of the plant to pro- 

 duction on the remaining shoots or spurs and causes them to produce 

 larger, if not more, fruits. In the section on Fruit Setting it is pointed 

 out that pinching back the growing shoots of the grape before blossoming 

 sometimes leads to a better setting. In most species continued thinning 

 out leads eventually to tall or wide spreading and "rangy" plants, plants 

 that require wider spacing in the orchard, that often make undue expense 

 in pruning, spraying and other care and that are unable to mature their 

 crops without a great number of mechanical supports. Judicious heading 

 back corrects these tendencies and promotes a compact type of growth 

 that, in these respects, is much more satisfactory. In fact it may be 

 stated that in general the main purpose of heading back is to control the 

 form of the tree, bush or vine — to train it. In practice this means that 



