26 PRINCIPLES AND PRACTICE OF PRUNING 



may tend to dry the active tissues, close the stomata and 

 otherwise reduce photosynthesis. 



It has been estimated that a vigorous Concord grape- 

 vine in good vineyard practice may expose to the light a 

 leaf surface of 10 to 12 square yards. Such an area has 

 been estimated as manufacturing about a pound of or- 

 ganic matter in a day during the active growing season. 

 From this calculation a hazy idea may be formed of the 

 work done by a vineyard or an orchard between, say, 

 April and October, and a hint may be gained as to the 

 importance of pruning and training so as to enhance the 

 process of photosynthesis, especially in fruit-bearing 

 plants. 



30. Growth and growth relations. — Though growth and 

 growth relations have been touched upon indirectly in 

 the previous chapter, it is necessary that certain of their 

 phases be discussed separately, because a true conception 

 of them is essential in pruning. From the agricultural 

 standpoint, growth is measured by yield ; from the stand- 

 point of the forester, usually b}^ size or development ; 

 from the standpoint of the ornamental horticulturist, by 

 beauty of specimen. These ends are all affected by va- 

 rious internal and external factors, such as (1) vitality, 

 heredity and food supply and (2) environment, moisture, 

 temperature, oxygen, character of food and light. 



31. Proof of growth. — Popularly, growth is evidenced 

 in flowering plants by increased weight and size, by de- 

 velopment from the seedling stage to maturity and the 

 quick production of stems, leaves, flowers and fruits. It 

 is dependent upon the formation and enlargement of cells 

 and may lead to strongly marked changes in both internal 

 structure and external form. The most evident of these 

 changes are (1) differentiation, due to structural changes 

 either within a single cell or a group of cells; and (2) 

 extension or increased length, often accompanied by 

 greater girth. 



