140 Principles of Flant Culture. 



sary to plant culture in arid regions, and may be profit- 

 ably employed at certain times in the great majority of 

 seasons in many localities where the annual rainfall would 

 satisfy the needs of crops, were it more uniformly dis- 

 tributed. 



235. Drying beyond a certain limit Kills Plant Tissues 

 by destroying in part their power for conducting water. 

 Care should be used to retain the normal moisture in 

 buds (394), cuttings (358), and cions (386) and in the 

 roots of plants lifted for transplanting. 



Section IV. Plants as Affe( 'ted by TJnfayoeable 



Light 



A — By Excessive IjKiHT 



236. The Unobstructed Rays of the Sun are often Injur- 

 ious to young seedlings, to unrooted cuttings and to plants 



recently transplanted. 

 It is difficult to sepa- 

 rate the influences of 

 light and heat, since 

 the heat is usually 

 greatest where the sun' s 

 rays are brightest; but 

 bright light probably 

 stimulates transpiration 

 FiG.67.1^b'Z^^nBea for shading (75) independent of heat 



cold-frames and tender plants in the open and thuS tcuds tO CX- 



ground. (After Bailey). haust the plant of water. 



Various devices are used to break the force of the solar 

 rays. In out-door culture, screens of lath (Figs. 61, 62)^ 

 cloth or brush (Fig. 63) are often placed oYcr beds con- 



