IMPORTANCE OF WATER 129 



the injurious effect of transitory frosts is by decreasing the power of 

 radiation of the air. The air is either heated directly by means of 

 portable stoves, or smudging is employed. In the latter case pitch, 

 manure, dry leaves, etc., are burned in different parts of the orchard. 

 These combustibles produce thick clouds of smoke which gather 

 over the plants and thus decrease radiation. In order to prevent 

 the damage wrought by prolonged winter frosts, one has to cover 

 carefully sensitive plants with snow, straw, dry leaves, etc. Where 

 this is impossible, as for instance with overwintering plants grown 

 under field conditions, the only means of protecting is the selec- 

 tion of more hardy varieties. A detailed investigation of such 

 hardier varieties has shown that in winter they possess a higher 

 sugar content, less water, and a higher water-retaining power of 

 their colloids. 



