PLANT PHYSIOLOGY AS RELATED TO PRUNING 



25 



branches and the arrangement of the branches (Fig. 20). 

 Plants growing alone in the open develop leaves uni- 

 formly on all sides (Fig. 20), those growing in groups 

 lack leaves where other plants compete with them for 

 light (Figs. 18, 22, 23). The same is true of branches and 

 twigs in trees. Hence the importance of pruning so as 

 to admit ample light to supply leaves on the interior twigs 

 and branches of trees and shrubs ; for when such parts 

 are poorly lighted they cannot function properly. In 

 many cases — for instance, interior twigs on young (three 

 to five-year-old) peach trees improperly pruned — they die 

 for lack of light. 



29. Photosynthesis is the process whereby the chloro- 

 phyll is able, in the presence of light, to construct organic 

 foods from the crude materials contained in the soil 

 water and the air. Though 

 light, carbon dioxide and 

 water are essential, the proc- 

 ess is influenced by tempera- 

 ture and other factors unnec- 

 essary to discuss. The intensity 

 and the quality of light bear 

 a more or less complex rela- 

 tion to food manufacture. 

 When the temperature is fa- 

 vorable, the capacity of plants 

 to oroduce finished plant food fig. i9— cross section of a 



• • 1 ^ iU • LEAF 



IS proportional to the in- ^^^^^^ ^^^ |^^^,^^ epidermis with 



tensitV of the lieht up to the stomata, a; air cliambers, f>, orimer- 



-^ . . , 1 cellular spaces; cross section of leaf 



limit of the available carbon vein, c, in the parenchyma or soft, 



,. . < , , T thin-celled tissue. 



dioxide and water. In sup- 

 port of this statement instances may be suggested of 

 plants which stand the full glare of the sun and of 

 others whose maximum is reached in partial shade. Too 

 strong emphasis, however, must not be placed on this 

 intensity factor, because increased transpiration of water 



