EXTREMES OF HEAT AND COLD. 



471 



colder climates for the exercise of their normal functions, but 

 they will also gcnerall3- sustain much higher degrees of heat with- 

 out injury. The differences of temperature in favor of tropical 

 plants are not, however, always ver}- marlved. 



The following table ' indicates sufliciently the highest tempera- 

 tures which a few common plants can bear. The line at the top 

 shows what were the immediate surroundings of the plants ex- 

 perimented upon ; the columns marl«:ed A show tlae highest 

 temperatures short of proving fatal ; tliose marked B, the low- 

 est fatal temperatures. The plants were exposed to the high 

 temperatures from fifteen to thirtj' minutes. 



1229. After a plant has been subjected to too high a tempera- 

 ture, its foliage wilts and soon becomes dry ; and its leaves, 

 having once taken on a scorched appearance, are unable to 

 recover their turgescence. it may happen, however, that the 

 injury does not proceed so fai as to affect the latent or even the 

 partially developed buds ; if this is the case, partial recovery 

 takes place through their unfolding. The curious fact^ that 

 many algse can resist very high temperatures has been already 

 adverted to (see 566). 



1230. Effects of cold upon the plant. Certain plants are seri- 

 ously injured by low temperatures which are considerably above 

 the freezing-point of water, but these are exceptional cases. 

 Most northern plants can readily endure cold, provided tlieir 

 tissues are not frozen. 



Frost produces very different effects upon different plants. In 

 some of our familiar spring plants the leaves may be frozen and 

 thawed without apparent mischief, but in general the thawing 

 must take place slowly ; if it proceeds rapidly, the plant may be 



1 De Vries: ArcWves Neerlaiidaises, v., 1870. 



2 Consult also American Journal of Science and Arts, xliv., 1867, p. 152. 



