256 Principles of Plant Culture. 



form a congenial field for certain destructive fungi 

 (321), that having once gained entrance, sooner or 

 later destroy the heartwood of the whole trunk, thus 

 greatly weakening it and preparing the way for the 

 final destruction of the tree. 



420. Objects of Pruning. If intelligently performed, 

 pruning has one of four objects in view, viz. : 



a To change the form of the plant, as to outline or 

 density (formative pruning}. 



b To stimulate development in some special part, as 

 to promote the growth of wood or the formation of 

 flower-buds (stimulative pruning.} 



c To prevent some impending evil to the plant, as 

 to arrest or exclude disease (protective pruning). 



d To hasten or retard maturity (maturative prun- 

 ing). 



A FORMATIVE PRUNING. 



This aims to regulate the form of the plant with 

 reference to outline or density, or to strength of stem. 

 Pruning for outline includes pruning (a) for symmetry 

 or picturesqueness ; (b) for stockiness or slenderness. 



421. Pruning for Symmetry aims to develop in the 

 plant a head that is symmetric with reference to its 

 trunk. The general principle involved is the suppres- 

 sion of growth in all parts that tend to grow beyond 

 the lines of symmetry (Fig. 156). This is best accom- 

 plished by pinching (416 a) during the growth period, 

 thus economizing the plant's energy; but when the 

 pinching has been neglected, the shoots that grew out 



