546 FREEZING AND BURNING. 



fall of temperature in the soil, the absorbent activity of the roots is so restricted 

 that the loss of water by the leaves through evaporation can no longer be replaced. 

 The leaves then become flaccid, shrivel and dry up, are blackened, and look 

 exactly like leaf-structures which have been killed by frost. It can be demon- 

 strated by a very simple experiment that the cause of death is only the fall of 

 temperature in the soil. If on autumn nights, when the temperature falls to + 1" 

 or + 2°, " very sensitive " Coleus plants in pots are brought from the warm green- 

 house into the open, the pots not being protected against cooling, these plants dry 

 up even the next day. If, on the other hand, the plants are sunk in warm sawdust 

 over which cotton- wool is strewn, and thus care is taken that the temperature of 

 the earth in the pots does not sink below -f 7°, then the Coleus does not dry up, and 

 generally sufiers no harm even although the temperature of the air and of the air- 

 surrounded leaves should fall during the night to -fO'5°. Since the conduction of 

 water to the transpiring leaves is sustained by the warmth of the soil, these leaves 

 may be protected from the so-called " freezing " even when they cool down to 4- 0"5°. 



Do means also exist by which plants may be protected from actual freezing? 

 To this question the answer follows naturally from the above discussion about the 

 real nature of freezing. If the plants in question can be hindered from assuming 

 that temperature in which their protoplasm is killed, then, of course, a protection 

 against freezing may be afforded. Usually bad heat-conductors are used as protec- 

 tive agents. The plant organs to be protected are clothed with dry straw and twigs, 

 or covered with dried foliage. In regions with continental climates vines are pro- 

 tected against freezing by surrounding the lower portions of the stock with earth. 

 Often plants are also protected by heaping up snow, and gardeners very generally 

 use snow as an excellent protection against freezing. As a matter of experience 

 numbers of plants perish with us during those winters in which no snow falls, 

 while they survive without injury the coldest periods of winter when the snow is 

 abundant. Many species of shrubs and low trees, of which only the lower half is 

 snowed up, while the upper half rises above the surface are found after severe 

 winters to be frozen from the apices of the branches down to the level to which 

 the snow has reached. This happened, for example, in the Vienna Botanic Gardens 

 (1880) with several young trees of the deodar {Gedrus Deodora), with the bushes of 

 Fontanasia jasminoides, and with shrubs of many species of jasmine and indigo. 

 But all these protective agents, twigs, straw, leaves, earth, and snow, fulfil their 

 function only in neighbourhoods where the cold period is of comparatively short 

 duration. In reality they ward off only the first onset of cold, and their principal 

 use lies in the fact that the radiation of heat from the covered portions is retarded. 

 In long and continuous cold the temperature of the coverings not only gradually 

 sinks, but finally that of the covered bodies also, and in Yakutsk a plant whose 

 protoplasm is killed at —10° can no longer be protected even by the thickest cover- 

 ing of straw, leaves, or earth. 



Moreover, in nature we can only speak conditionally of a natural protection 

 against freezing, and only in those regions where periods of great cold alternate 



