FKOZKN I'LAXT-ORGANS. 501 



humus, dead leaves, etc., than on bare soil, though in the 

 former case the frost remains longer. 



It. Density of Sfock. 



Natural regeneration-areas as well as plantations and sow- 

 ings under a shelterwood, which reflects back the radiated 

 heat towards the ground, whilst the crowns of the shelter 

 trees prevent rapid changes of temperature, suffer much less 

 from frost than sowings and plantations in the open. Young 

 plants suddenly exposed by the removal of a shelterwood are 

 highly susceptible to damage by frost, and large areas of 

 spruce 6 feet in height may i)e thus killed. 



Damage by frost is much less in well-stocked woods than 

 where blanks or thinly-stocked places occur. Unrestricted 

 radiation of heat and non-circulation of the air expose such 

 places to frost ; in the same way young growth surrounded by 

 tall woods is often frozen. 



/. Stall' of I lie Woatltor. 



The clearer the sky during day-time and the brighter the 

 night, the greater is the danger from frost, especially with an 

 east wind. It seldom freezes with a cloudy sky, as then the 

 heat radiated fi-om the ground is reflected back again by the 

 clouds. Late frosts accompanied by rime are more dangerous 

 than black frosts, as the coldness of the air is still further 

 increased by the evaporation of the frozen dew. In a pro- 

 longed frost, accompanied by cold dry winds, the frozen iwigs 

 may be dried up and killed. A wet autumn generally 

 increases the subsequent bad effect of the winter's cold. 



As a rule, in Central Europe, all danger from frost is over 

 by the middle of May, but exceptions may occur, and in 181)2 

 the grass-temperature at Coopers Hill, in Surrey, from the 

 13th to the 16th June, varied between 25" and 32' F., 

 so that potato-shoots and bracken were frozen and killed 

 in the neighbourhootl. In mountainous regions, late frosts 

 are to be feared till July. In the north of India, night- 

 frosts may occur, on clear nights, from October till the 

 end of March, and they cause considerable damage to 



