The Temperature Relation 



409 



cause drying out, and the structures which are. ordinarily 

 assumed to be protective against cold or heat are in reality 

 serviceable in preventing loss of water. The delicate 

 young buds of the peach or other deciduous trees may be 

 inclosed by bud-scales, hairs, and resins ; nevertheless, 

 such buds promptly freeze solid when the temperature 

 falls below the freezing-point of the cell-sap, or the point of 

 supercooling. The trunk of the tree is, of course, pro- 

 tected in a way by thick bark, yet so far as the entrance of 

 cold or loss of heat is concerned this protection is insignifi- 

 cant. 



248. Irritable response. Through growth movements 

 toward or away from a source of heat, plants commonly 

 exhibit the capacity for irritable response (positive and 

 negative thermotropism) with respect to temperature ; but 

 this response is of 



little practical sig- 

 nificance, except as 

 further evidence of 

 the paratonic rela- 

 tions of the organism. 

 Thermonastic move- 

 ments also occur, but 

 this general class of 

 phenomena is dis- 

 cussed in section 306. 



249. Freezing. 

 Some of the results 

 of freezing deserve 



careful consideration. Fl . G - I 09 - Froze ^ f , Fri r ?J aria J 



. ing ice-masses (stippled). [After Muller- 



It is well known that Thurgau.] 



