110 PRINCIPLES OF PLANT CULTURE 



increased by an overheated atmosphere that the roots 

 are unable to supply the plant with sufficient water, hence 

 the cells become partially emptied and the foliage droops. 

 Herbaceous plants in an overheated greenhouse or hotbed 

 are sometimes so prostrated from excessive loss of water as 

 to appear dead, but unless the heat has been sufficient 

 to destroy their protoplasm, or the heated period has been 

 protracted, they will recover when normal temperature 

 and water supply are restored. 



182. Effect of heat on evergreens. — 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 atmospheric temperature, especially if 

 accompanied Ijy a drying wind, may so far reduce the water 

 in the leaves of e\'ergreen trees as to cause death of the 

 foliage and even of the trees themselves. This most fre- 

 quently 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 : (o) means that 

 favor prompt thawing of the ground, as thorough drainage 

 and not too close planting ; (b) 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 providing wind- 

 breaks which tend to retain the snow (203). The prac- 

 tices are subject to considerable modification. 



183. High temperatures. — A temperature of 122° F. is 

 fatal to the protoplasm of most land plants-. Aquatic 

 plants and the more watery part of land plants perish 

 at a somewhat lower temperature. Watery fruits, as 



