326 Plant Physiology 



191. Cell division and respiration. It is obvious that 

 the growth of the embryonic cell in protoplasm often leads 

 to a climax of energy-release in the complex activities of 

 nuclear and cell division. When growth of the cell does 

 not lead to division, or multiplication of kind, it is usually 

 a progression towards differentiation, a process likewise 

 involving abundant metabolic changes and energy-release. 

 It is not strange that respiration in healthy organs is, in 

 general, a measure of growth intensity. 



192. The relation of pruning to growth. Pruning, as 

 applied to trees, shrubs, and vines, is a practice which has 

 as its chief ends a regulation of growth and fruiting, and 

 a shaping or training of plants. Either one or the other 

 of these ends may be purely incidental, but the process 

 is most important as a thinning of the fruit buds, and for 

 the regulation and distribution of new wood. The prac- 

 tice must vary with the species of plant, and with the local 

 ideas of proper size and shape. Properly performed, it is 

 physiologically rational, and the world-wide development 

 of the practice attests its effectiveness. Pruning should 

 not be regarded usually as a special form of forcing for 

 fruit production. 



In trees the leaf buds often develop most abundantly at 

 the tips ; that is, at the periphery of the entire tree, so that 

 the tree grows as a constantly enlarging shell. There are 

 many more buds produced on the periphery than could 

 possibly be developed profitably. Ordinarily, many more 

 begin to develop than could succeed. Pruning is needed 

 to suppress some buds, and to permit others to grow more 

 vigorously. It is also needed with certain fruits in order 

 to cut out and restrict large branches, so that light may 



