PLANT PHYSIOLOGY AS RELATED TO PRUNING 31 



moist, late summer or early autumn following a more or 

 less protracted drouth. It may also be due to excess of 

 nitrogenous plant food and an ample water supply in the 

 soil. Such growths are usually undesirable because they 

 are developed from buds which should continue dormant 

 until the following spring and also because they usually 

 do not have sufficient time to ripen fully before winter 

 sets in. Consequently much of the growths so produced 

 must be pruned off, to the greater or lesser loss of wood 

 that if grown in normal time would be valuable for fruit 

 production or other purpose. 



Hence cultural practices, such as tillage, drainage, fer- 

 tilizing, cover and inter-cropping, have a direct bearing 

 upon pruning, because these affect more or less definitely 

 the production of wood. In many kinds of trees, es- 

 pecially apple and pear, leaves appear to be borne in 

 clusters on very short twigs called spurs. This phe- 

 nomenon is due to the suppression of the internodes and 

 the consequent shortening of the axis. Sometimes the 

 reverse of this case occurs on a spur, as when a bud, which 

 might be expected to continue the slow extension of a 

 spur, suddenly elongates into a slender twig such as 

 shown in Fig. 36. 



36. The relation of leaves to light intensity is one of 

 the most commonly observed of plant growth phenomena. 

 Everyone who has had anything to do with plants, es- 

 pecially those grown in dwellings, knows that leaves and 

 leafy shoots "turn toward the light." This is the more 

 marked as the room is poorly illuminated ; the plant parts 

 capable of doing so soon bend toward the window or other 

 light source. Similar effects may be seen in the open. 

 The explanation for this phenomenon is that the parts 

 which thus change position do so in order that the 

 chlorophyll may secure the maximum illumination and 

 thus function to best advantage. 



Broad-leaved plants generally form a more or less com- 

 plete mosaic of green. Especiallv good examples of this 



