INJURIES DUE TO ATMOSPHERIC INFLUENCES 299 



being unprovided with a properly developed epidermis, wither or 

 easily succumb to other influences when the plants are again 

 fully exposed to the light, and are incapable of developing into 

 normal shoots. 



The laying of cereals is the result of the shading of the lower 

 internodes in consequence of thick seeding or heavy manuring. 

 The restriction of light that results from drilling seed thickly 

 stimulates spruces, pines, and other plants to make increased 

 height-growth, but this is secured at the expense of the lateral 

 shoots and the health of the plants. 



MECHANICAL INJURIES 



Reference may here be made in a few words to such 

 mechanical injuries as are due to atmospheric precipitations 

 and violent gales, and especially as these often lead to other 

 diseases. 



Flowers and leaves are damaged by heavy hail, which may 

 also severely injure the cortex of trees, especially when the rind 

 is smooth. At the 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 

 unfrequently 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 in the beech (Fig. 39, page 93). Larches, 

 too, are often similarly infected by Peziza Willkommii. 



There is not much to be said about the damage that is induced 

 by snow-crushing. For obvious reasons, this occurs almost 

 exclusively in woods of evergreen conifers, where it takes the 

 form either of the breaking off of the tops and branches, or of 

 the fracturing of young poles. It may be worth noting, however, 

 that wounds are very often formed at the base of branches 

 which are bent down by a load of snow. Should the ground be 



