6 PROTECTION OF WOODLANDS. 



Under Early or Autumn Frosts are understood those occurring 

 early in autumn, before the young shoots have had time to harden 

 and become woody, which cause the death of the unhardened por- 

 tions ; they seldom kill the whole plant outright, the effects being 

 usually confined to the portions actually damaged by the early 

 frosts, i.e., the summer shoots in particular. The leaf -shedding 

 of Scots Pine seedlings (see par. 35) is also by many considered 

 to be the result of early autumnal frosts. 



The cause of plants being frost-bitten during the period of active 

 vegetation is explainable in the withdrawal of water from the 

 cells by the frost, the water quitting the cells and passing into 

 the intercellular spaces, so that the power of tension (turgor) of 

 the tissue becomes lost, and the portions affected wither and die 

 off when a thaw sets in. Whether these are actually killed by the 

 frost, or only die after thawing takes place, is a question not yet 

 satisfactorily answered ; for, although the latter has hitherto been 

 the view generally taken, Dr H. Miiller has recently adduced 

 strong arguments in favour of the former hypothesis, supporting 

 his view in favour of this by the fact that in hundreds of experi- 

 ments he has never been able to save, by means of very gradual 

 thawing, portions of plants that would have been killed by rapid 

 thawing. 



Frosts occurring in Winter during the period of complete rest 

 from active vegetation do not, as a rule, do much damage to our 

 indigenous trees or to such exotics as have become acclimatised, 

 although we not infrequently see somewhat incompletely hardened 

 shoots of the previous year, the summer shoots especially, killed 

 off, or the roots of young Oaks die in consequence of long-con^ 

 tinued hard frost when the soil is unprotected by snow, or even 

 the killing off of older plants and steins, as could frequently be 

 noted in the hard winter of 1879-80. Here again, too, the with- 

 drawal of more than a certain limited quantity of water from the 

 cellular tissue is regarded as the cause of the damage. At the 

 same time, a drying up of the foliage of coniferous trees may be 

 the consequence of continuous cold, in consequence of the rapid 

 transpiration of water through the needles on sunny wintry days, 

 without the possibility of the evaporated water being replaced by 

 the frozen woody tissue of the stem ; this phenomenon may not 

 infrequently be noted on trees exposed to the sun at the edge of 

 woods, and on such as stand freely exposed to the southern side. 



