122 AGRONOMY 



to avoid the heat, while in various other plants the chloro- 

 plasts are able to change their position in the cell. 



Need of light. Light is necessary for the existence of every 

 independent plant, since the energy for food making is derived 

 from this source. The general effect of light upon plants, how- 

 ever, is to inhibit growth, and chlorophyll itself, the very sub- 

 stance by means of which the chloroplasts turn sunlight into 

 useful energy, is broken down in strong light. Bacteria soon 

 die when exposed to direct sunlight, and many of the higher 

 plants cannot live long under such" conditions. These latter 

 grow in forests, ravines, and other secluded places and are 

 known as shade plants. Ferns and many of the plants that 

 bloom in early spring are shade plants. Many of our forest 

 trees are essentially shade plants when young. Absence of 

 sufficient light, however, is quite as bad as too much. In in- 

 sufficient light the formation of wood cells entirely ceases. 

 As in the case of temperature, the plant is balanced between 

 two harmful extremes. 



Effects of lack of light. Sun-loving plants grown in deep 

 shade are deficient in chlorophyll, and have small leaves and 

 weak stems which are greatly elongated, or " drawn." The 

 flowers are also paler and the fruit scanty and lacking in flavor. 

 Aromatic plants have their aromatic properties lessened when 

 grown in shade. Plants may receive too little light by being 

 planted so closely together that they shade one another, or 

 where a subsequent growth of weeds springs up and over- 

 shadows them. Fruit trees and flowering plants, generally, 

 may fail to form flower buds unless pruned sufficiently to let 

 the light into the mass of foliage. A large number of woody 

 plants have the faculty of self-pruning. This is regarded as 

 a response to insufficient light. The cottonwood is one of the 

 best-known trees with this habit. In winter the earth beneath 

 old trees is thickly strewn with twigs a foot or more long, cut 

 off by the tree as smoothly as the leaves are. - 



