90 THE NATUEE AND WOEK OF PLANTS 



species are quite blackened and shrivelled, while 

 others still appear bright and green and remain so 

 until the actual approach of winter and many frosts 

 have been endured. This leads at once to the con- 

 clusion that separate species have different powers of 

 resistance to cold. 



It is seen at once that a temperature sufficient to 

 freeze water does not kill all species. Then again 

 some species, such as the melons, coleus, tobacco, and 

 tropical plants are killed by temperatures of two 

 degrees above the freezing point. Apple leaves are 

 killed by a temperature of two to six degrees below 

 the freezing point, cabbage five to seventeen be- 

 low, peaches two or three below, tomatoes one below, 

 wheat one below, strawberries two to four below, 

 while the grasses of the Arctic regions endure temper- 

 atures of eighty and ninety below the freezing point. 



The water in some species may be frozen without 

 damage to the protoplasm, but if the frozen speci- 

 mens are brought into a warm room and thawed 

 quickly, the shock of the sudden change will kill 

 them. It is for this reason that frozen specimens 

 may be sometimes thawed without damage in water. 

 The use of the water also prevents the specimen 

 from drying out. 



