292 DISEASES OF TREES 



impunity, whereas if the leaves have appeared they suffer from 

 a few degrees of frost. In this case the view is undoubtedly 

 correct that death from frost only occurs with the thaw. When 

 plant-tissue is frozen during active growth, it exhibits the 

 conditions that have already been described. Should the plant 

 thaw very gradually, the water is absorbed by the walls and 

 contents of the cells at the same rate as it is formed from the 

 ice-crystals by the gradual accession of heat, so that when the 

 cells have attained the temperature -at which chemical processes 

 are possible the normal conditions of imbibition have also 

 been again restored, and the metabolic processes which were 

 temporarily suspended are resumed under the influence of the 

 higher temperature. The case is different, however, when the 

 frosted parts of plants are rapidly thawed, as occurs, for instance, 

 when they are brought into a warm room, or are touched by the 

 warm hand, or are suddenly warmed by the sun. The rapid 

 accession of heat induces the ice in the intercellular spaces to 

 thaw rapidly, and the ice-water, being but slowly absorbed by 

 the cell-walls and protoplasm, flows into the intercellular spaces, 

 and drives out the air, with the result that leaves which are 

 suddenly thawed become translucent. The normal conditions of 

 imbibition have not been restored when the chemical processes 

 start afresh under the influence of the rise in temperature. 

 Instead of these processes assuming the normal features of 

 metabolism, they lead to chemical decomposition in the com- 

 paratively dry and withered tissues ; in other words, they induce 

 death from frost. It is therefore emphatically to be recom- 

 mended that plants affected by late frost should be protected 

 against a too rapid thaw. 



It often happens, even in the case of our indigenous trees, 

 e.g. the oak, that after a cold wet summer the vigorous Lammas 

 shoots have not ceased growing when the first early frost 

 appears. Exotic trees, whose vital processes demand more heat 

 for their normal maintenance than our climate has to offer, find 

 themselves every year in an unprepared condition on the advent 

 of winter. The youngest organs of the annual shoots have not 

 completed their development (and especially is this the case 

 when growth in height continues till the latter part of summer, 

 as happens with Ailanthus, &c.), the youngest elements of the 



