72 PLANTING 



southern than on northern aspects. Now, inasmuch as the 

 decay of leaves and humus, which hold moisture, as it were 

 like a sponge, is very much more rapid on southern aspects, 

 it follows that this loss of humus is accompanied by a corre- 

 sponding diminution in the available amount of moisture for 

 the crop. It is this lack of sufficient moisture which is 

 usually responsible for the less vigorous growth of trees on 

 southern aspects than is evidenced on northern aspects. 



The prevalence of, and the damage caused by, late spring 

 and early autumn frosts, especially to young crops, is, to a 

 very great extent, governed by the aspect of the locality ; or 

 more correctly, by the aspect when considered along with the 

 altitude. These frosts are a more frequent cause of the 

 failure of young plantations than is commonly imagined. 



The Influence of Aspect and Altitude on Frosts. In 

 discussing this question, it is necessary to remember that 

 these late spring and early autumn frosts are very local 

 indeed. They occur usually on still, cloudless nights, when 

 the air is very dry, and especially after a hot, sunny day. 

 The area affected is usually very circumscribed ; and it is 

 the lowest land in the locality, especially if wet, that is 

 affected, to which the cold air sinks, and from whence it 

 cannot find an outlet ; whereas the land a few feet higher 

 will, as the cold air descends, and is therefore in motion, often 

 escape. Hence it is that these frosts are far more common 

 on southern aspects, where the air is dry, and at relatively 

 low altitudes, where the cold air remains stationary. Then 

 again, they are more common on stiff, wet land, which a hot 

 sun has chilled by evaporation, than on well-drained land. 

 They often extend over a large area of flat land, as, rela- 

 tively, it is low-lying land ; whereas on undulating and on 

 sloping land, the frost is confined to the valleys, dells, or 

 coombes. Another very important point to remember is, that 

 there is a greater liability to these frosts on maiden land 

 covered with grass and weeds, than on land that is perfectly 

 clean, or which has a good layer of humus covering it. For 

 in this latter case, the rapid evaporation of water and radiation 

 of heat is retarded ; whereas if there be a surface covering 



