EXTREMES OF HEAT AND COLD. 



471 



colder climates for the exercise of their normal functions, but 

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

 out injury. The differences of temperature in favor of tropical 

 plants are not, however, always very marked. 



The following table 1 indicates sufficiently the highest tempera- 

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

 shows what w r ere the immediate surroundings of the plants ex- 

 perimented upon ; the columns marked A show the highest 

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

 est fatal temperatures. The plants were exposed to the high 

 temperatures from fifteen to thirty 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 ma}* happen, however, that the 

 inj in*}' d es n t proceed so fai is to affect the latent or even the 

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

 takes place through their unfolding. The curious fact 2 that 

 mairy algse can resist very high temperatures has been alreacVy 

 adverted to (see 566). 



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

 ously injured by low temperatures which are considerable' above 

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

 Most northern plants can readily endure cold, provided their 

 tissues are not frozen. 



Frost produces very different effects upon different plants. In 

 some of our familiar spring plants the leaves ma3' be frozen and 

 thawed without apparent mischief, but in general the thawing 

 must take place slowly ; if it proceeds rapidly, the plant ma3 T be 



1 De Vries: Archives Neerlandaises, v., 1870. 



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



