DAMAGE CAUSED BY LOW OR HIGH TEMPERATURE. 15 



B. HEAT. 



10. Damage done by Heat. 



Under heat is understood an unusually high degree of warmth 

 developed by the sun. It occasions damage directly only in the 

 case of scorching or sun-burn, to be considered later, but has also 

 an indirect disadvantageous effect owing to drying up the soil 

 through exhaustion of its moisture. 



Warmth, even to a very considerable degree, is beneficial to 



the activity of vegetation so long as a sufficiency of moisture 



is held in the soil, or is obtainable either from atmospheric 



precipitations or by artificial watering. If, however, the necessary 



supply of moisture is wanting, the soil becomes dried up, and the 



plant, whose transpiration through the leaves is greatly stimulated 



by atmospheric warmth and dry winds, is unable to obtain the 



amount of moisture from the soil necessary for the maintenance 



of equilibrium between evaporation and requisite amount of 



moisture throughout the plant, so that a disturbance sets in 



prejudicial to the latter ; leaves and flowers first wither and 



droop, then turn brown and fall off; seedlings, young plants, 



and with continuous drought even older plants, become dried 



up and . die off ; germinating seed gets dried up, and fruits that 



may have set on the trees either become unproductive or are 



shed immaturely, as often happens in the case of Oak and Beech. 



Older trees naturally suffer least, although in very dry seasons 



there is a distinct falling off in the increment for the year, 



[whilst the effect of intense solar warmth not infrequently shows 



! itself in the production of immature autumn tints in the foliage. 1 



j Considerable numbers of trees often die off in consequence 



I of the warmth, and in the year following any abnormally dry 



I summer there is always a larger fall of dead wood from the 



crowns. 



As secondary effects of continuous heat and drought, there may 



j l During a tour in the forests of the south-east portion of the Bavarian highlands, 

 |to wards the end of August 1892, I had abundant opportunity of noting how soon 

 jthe intense and abnormal heat of the previous fortnight had affected many parts of 

 (hillsides, particularly southern exposures, on which the soil was at all wanting in 

 depth. Beech had in general suffered most, but Spruce had often been killed out- 

 right owing to the exhaustion of the soil-moisture. In towns the Horse-chestnuts 

 iwere also shedding their leaves as if in autumn. Trans. 



