136 PRINCIPLES OF PLANT CULTURE 



leaves (101) where it passes off by transpiration (74). 

 Weeds, therefore, rob crops of moisture (336). 



233. Irrigation, i.e., the extensive watering of out-door 

 plants, is the final remedy for drought. It is necessary to 

 plant culture in arid regions, and may be profitably em- 

 ployed 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 distributed. 



234. 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. 



