492 PROTECTION AGAINST FROST. 



ii. Splitting the stems of trees. 



iii. Causing canker in stems of trees. 



iv. Uprooting young plants. 



SECTION I. FROZEN PLANT-ORGANS.* 

 1. External Appearance of Injured Plants. 



Frozen plants, or organs of plants, become soft, flexible and 

 hang down or wilt. When dead they eventually turn brown 

 or black. These outer signs result from the reduced tension 

 of the tigsues injured by frost, and from their inability to fulfil 

 their proper functions. Foliage frequently falls prematurely 

 owing to early frosts, a film of ice forming at the base of the 

 petiole, as in robinia and elder, which may become leafless 

 in a few days, the fallen leaves remaining quite green and 

 apparently unaffected by the frost. In other trees, as horn- 

 beam, beech or oak, the frozen foliage may die and turn 

 brown, and remain on the tree until the buds swell in the 

 spring, the normal autumnal leaf-fall being prevented by the 

 fact that the leaf is killed before the usual layer of cork, which 

 causes defoliation, has formed at the base of the petiole. 



M. Her relates ("Rev. des E. et F.," July, 1897, p. 424) 

 that in a frost in February, 1895, in the Hautes Vosges, near 

 the Lake of Longemer, young silver-fir attacked by frost did 

 not lose their lower branches, which were protected by snow, 

 while their middle branches, above the snow, were killed. 

 The budp of the two or three highest verticils remained 

 dormant for a year, producing no shoots in 1895, and some 

 of them not even in 1896, whilst the terminal bud produced a 

 leader in the summer succeeding the frost. Hence it appears 

 that organs may be affected by frost, without being killed, 

 and that the most vigorous organs, such as terminal buds, 

 best resist frost. 



2. Explanation of the Action of Frost. 



Death by freezing is usually caused by late frosts, less by 

 early frosts. Winter frost rarely kills indigenous plants. 



* Goppert, Dr. H. R., " Ueber das Gefrieren, Erfrieren tier Pfhumni mid 

 Schutzmittel dagegen." Stuttgart, 1883. 



