IRRITABILITY. 135 



slowly they are thawed out, are killed after freezing, as the leaves of the 

 pumpkin, dahlia, or the tubers of the potato. 



282. It has been found that as a general rule when plants, or plant parts, 

 contain little moisture they will withstand quite high degrees of tempera- 

 ture, as well as quite low degrees, but when the parts are filled with sap or 

 water they are much more easily killed. For this reason dry seeds and the 

 winter buds of trees, and other plants, because they contain but little water, 

 are better able to resist the cold of winters. But when growth begins in the 

 spring, and the tissues of these same parts become turgid and filled with 

 water, they are quite easily killed by frosts. It should be borne in mind, 

 however, that there is great individual variation in plants in this respect, 

 some being more susceptible to cold than others. There is also great varia- 

 tion in plants as to their resistance to the cold of winters, and of arctic 

 climates, the plants of the latter regions being able to resist very low tem- 

 peratures. We have examples also in the arctic plants, and those which 

 grow in arctic climates on high mountains, of plants which are able to carry 

 on all the life functions at temperatures but little above freezing. 



For further discussion as to relation of plants to temperature, see Chap- 

 ters 46, 48, 49, and 53. 



