514 



CHAPTER II. 



PROTECTION AGAINST INSOLATION. 



INSOLATION may dry up seeds and young plants, or twigs and 

 branches of trees ; it may also scorch or crack the bark and 

 wood of trees. 



SECTION I. DROUGHT. 



1. Appearance and Cause of Injury. 



A period of more than 14 consecutive days without rain is 

 termed "absolute drought"; a "partial drought" is when 

 for 28 days the average daily rainfall is not more than 

 one-hundredth part of an inch.* 



Woody plants, and parts of them which have been dried by 

 the sun, have much the same appearance as if they had been 

 killed by frost ; blossoms, leaves, needles and young shoots 

 first wilt, then gradually turn brown and shrivel up ; they 

 finally fall off, although dried leaves may remain for a long 

 time hanging on the trees. 



This drying-up of leaves and shoots is the result of pro- 

 longed hot, dry weather, which abstracts much moisture from 

 plants and from the soil ; transpiration being thus greatly 

 increased, plants pass off more watery vapour into the air 

 than their roots can absorb from the soil, which, becoming 

 continually drier, is less able to meet their demands. The 

 action of heat on forest plants is therefore indirect, as the sun's 

 rays do not kill them directly. 



2. Damage done. 

 A. General Account. 



Owing to the want of the necessary moisture in the soil 

 usually caused by high degrees of heat, seeds may not secure 

 sufficient water for germination, and young seedlings may 

 be killed. In the case of older plants the foliage dries up and 

 falls prematurely, the younger shoots may be killed, and the 

 annual wood-increment considerably reduced. 



Drought at the commencement of the growing season reduces 



* "British Rainfall" (Ed. Stanford, i-ong Acre, 1906). 



