Plants as Affected ~by Heat. 119 



182. Evergreen Trees are sometimes Destroyed by 

 Untimely Warm Weather in spring. With a soil so 

 cool that the roots are inactive, a sudden rise of atmos- 

 pheric temperature, especially if accompanied by a dry- 

 ing wind, may so far reduce the water in the leaves of 

 evergreen trees as to cause death of the foliage and even 

 of the trees themselves. This most frequently happens 

 in the seed-bed, in compact nursery plantations, or 

 with recently transplanted evergreen trees. It is most 

 disastrous on poorly-drained clay soils that have a sunny 

 exposure, and at times when the ground is deeply frozen. 



The preventives to be observed are, a, means that 

 favor prompt thawing of the ground, as thorough drain- 

 age and not too close planting; &, means that prevent, 

 in a measure, exposure to the sun, as planting on a 

 northern slope or shading the trees (414) ; c, means 

 that tend to prevent deep freezing of the soil, as pro- 

 viding wind breaks which tend to retain the snow (203). 



183. A Temperature of 122 F. is Fatal to the Pro- 

 toplasm of most land plants. Aquatic plants and the 

 more watery parts of land plants perish at a somewhat 

 lower temperature. Watery fruits, as tomatoes and 

 gooseberries, and the younger leaves of deciduous trees, 

 are sometimes destroyed by exposure to the sun's rays 

 in hot weather. An occasional sprinkling of the plants 

 and of the soil about them will usually prevent this 

 result. 



184. Plants Under Glass should Not be Sprinkled 

 in Bright Sunshine. Drops of water upon the leaves 

 of plants often act as lenses in converging the rays of 

 the sun, and in a closed greenhouse or hotbed may cause 



