34 DISEASES OF CULTIVATED PLANTS 



Injury caused by hail. The amount of damage to leaves 

 and flowers caused by hail is well known. The fact that an 

 equal or greater amount of damage to the bark of trees, 

 especially when the rind is smooth, is occasioned by hail is 

 not so generally realised. Hartig states: 'At places where 

 the hailstones strike, the rind is crushed, or, it may be, 

 knocked off altogether. Although as a rule a callus very 

 soon forms over such wounds, still it not infrequently 

 happens that the injured portion of the stem dies. In young 

 spruce woods in the neighbourhood of Munich I found that 

 the leading shoots which were affected by hailstones died a 

 result doubtless due to the excessive evaporation from the 

 wood, which in many cases was stripped of its cortex on one 

 side of the shoot to the distance of about an inch. 



' It very frequently happens that the wounds caused by 

 hailstones form an entrance for parasitic fungi. The spores 

 of Nectria ditissima are specially apt to germinate on such 

 places, and to produce canker on the beech. Larches, too, 

 are often similarly infected by Peziza willkommii (=Dasy- 

 scypha calycina}.' 1 



In the winter of 1907 a pear-tree was badly injured by 

 hail, and the accompanying illustration shows portions of a 

 branch of the injured tree as it appeared during the winter 

 of 1908. Immediately after the storm the cortex was seen 

 to be bruised in those places struck by the hailstones, at a 

 later date those bruised portions died, contracted, and 

 separated more or less from the surrounding living tissue. 

 In some of the cracks thus formed various kinds of fungi 

 had established themselves. 



Injury by hail can generally be recognised by one side of 

 the branch only being damaged, the sheltered side remaining 

 sound. 



Hartig and Somerville, Diseases of Trees (Engl. ed.), p. 

 2 99 (i 8 94)- 



CHLOROSIS 



This term is used to express the gradual disappearance of 

 the green colouring matter chlorophyll, from leaves. In 

 some instances this loss of green colour extends over the 

 entire surface of the leaf, in others only patches disappear 

 here and there, giving the leaf a variegated appearance. 



