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and trees. The injury or death is in this case 

 caused by some change in the protoplasm brought 

 about by the low temperature in a way which is not 

 yet understood, and which varies for different species. 

 On freezing, ice usually forms in the intercellular 

 spaces, water being withdrawn from the cells. This 

 withdrawal of water and drying of the cell contents 

 appears to be the cause of death in some cases. As a 

 rule the rapidity or otherwise of the freezing or thaw- 

 ing has no specially injurious effect, the injury being 

 due to the low temperature, although we can 

 generally only see if a plant is dead or alive after 

 thawing. (This action must be distinguished from 

 that of indirect desiccation, (2) below, in which a 

 sudden rise of the air temperature may be very 

 injurious, by increasing transpiration from the leaves 

 while the roots are still inactive in frozen soil. ) The 

 temperature of plants may be very considerably 

 reduced by radiation which may result in the tem- 

 perature of the plant being as much as 8 C. below 

 that of the air. Radiation is reduced by fogs, 

 clouds, dust, or smoke, and the kindling of smoky 

 fires is often efficacious in preventing frost damage, 

 as also are artificial coverings of various sorts. 

 The temperature of the leaves, twigs, and more 

 delicate serial portions of plants chiefly depends 

 on the temperature of the surrounding air, and 

 hence the damage done by frost depends to a 

 great extent on the locality. The absence of air 

 currents is one of the most important factors which 

 affect the air temperature and hence low-lying 

 enclosed situations where there is little or no cir- 

 culation of air are particularly liable to frost damage. 

 Such are the grassy blanks so frequently seen in 

 Sal (Shorea robusta) forests, in which the circulation 

 of air is prevented by the dense wall of high Sal 

 forest surrounding them on all sides and where the 

 cold air collects and stagnates. In such places 

 all Sal shoots are killed down year after year by 

 frost, whereas small patches of Sal forest may often 

 be seen in the same locality untouched by frost, 

 owing to the open cultivated ground on all sides 

 oi it promoting the free circulation of air currents. 

 In these frosty localities the cold air collects like 



