l84 DISEASE IN PLANTS. 



during spring when there is no snow to protect the 

 plants. The soil freezes during the night, and 

 during the thaw next day water accumulates just 

 below the surface. The freezing is then repeated, 

 and, partly owing to the expansion of the forming 

 ice and partly to the mechanical effect of the ice- 

 crystals in the interstices, the surface of the soil is 

 lifted and draws the roots with it. During the 

 succeeding thaw the soil particles fall away from 

 the lifted root-fibres, and frequent repetition of 

 these processes results in such complete exposure 

 of the roots to the full sun that the plantlet falls 

 over and wilts. 



Exposure of roots is also sometimes effected by 

 winds displacing sandy soils liable to shifting in 

 dry weather, and the resulting wilting of the plants 

 thus exposed at their roots may be supplemented 

 by damage due to the repeated impact of the 

 wind-driven sharp grains of sand, which act like 

 a sand-blast and erode the tissues. 



In many of the cases given above the principal 

 result is the weakening or destruction of the chloro- 

 phyll action. This means a loss of carbohydrates 

 sugars, starches, etc.- and in so far a starvation 

 of the plant. The injurious effects are quantitative 

 and cumulative : if large areas of foliage are con- 

 cerned, or if the effect lasts a long time, the plant 

 suffers from loss of food, and may die. In those 

 cases where the effect is due to the cutting off 

 of supplies at the roots, and where the yellowing 

 is a secondary symptom, the disease is more 

 general in character, and recovery is often im- 



