DAMAGE CAUSED BY LOW OR HIGH TEMPERATURE. 19 



circulation of air (aeration) is always much greater than in heavy, 

 tenacious soil, thereby directly affecting the precipitations of the 

 moisture that is always present in the atmosphere. 



On areas where sowing has been carried out, the moisture 

 necessary for germination can best be provided by deepening the 

 bands on which the seed is sown, and forming a ridge on the 

 south side with the material removed, or by protecting seed-beds 

 with a covering of moss, branches, straw, or protective frames ; 

 young plants, especially the more sensitive young seedlings, may 

 best be protected by means of branches stuck in the ground, 

 by frames, or by layers of moss spread between the rows. 



The direct increase of moisture by watering or irrigation is 

 seldom practicable on any large scale. 



Watering can, of course, only be carried out in temporary or 

 permanent nurseries, the seed-beds being watered both before and 

 after the germination of the seed ; it is, however, less frequently 

 applied when the seedlings or transplants are somewhat older. 

 It is always a more or less costly measure, especially when water is 

 not obtainable in the nursery itself, and except in urgent cases, 

 as, for example, long-continued drought in regard to such species 

 as Elm or Alder, which are very sensitive and occasionally may be 

 urgently in need of moisture, it does not take place so long as it 

 is possible to do without it. 



Where circumstances are favourable the irrigation of nurseries 

 has certainly its advantages, and is carried out here and there in 

 the same manner as in horticulture ; but in general it is not appli- 

 cable, partly owing to the cost, partly owing to the usual want of 

 the necessary supply of water. 



The irrigation of dry hillsides has recently, whilst so much 

 attention has been paid to the maintenance of water-supplies 

 in woodlands, been recommended wherever circumstances permit 

 of it, and has actually received a certain amount of practical atten- 

 tion here and there, use being made of the surplus moisture 

 obtained by drainage of higher lying, damp plateaux, and of water 

 collected in the ditches along the sides of roads, which, in place of 

 being conducted as formerly by the shortest way to the nearest water- 

 course, is carried sidewards along the slopes into horizontal ditches, 

 whence it is allowed to percolate into the soil. According to Ney, 

 the damming-up of smaller brooklets, and the leading-off of the 

 water sidewards, is also a perfectly practicable measure. 



