TEMPER ATUEE, LIGHT, AND MOISTURE 121 



is greater than the roots can supply, and the drooping of the 

 foliage ensues. If this continues long enough, it may cause 

 the death of the plant, though in bright sunshine a great 

 many plants wilt during the hottest part of the day and 

 revive as the temperature falls. The watery parts of plants, 

 especially the fruit and young leaves, being less resistant to 

 heat than the rest of the plant, are often destroyed by exposure 

 to the hot sun. Evergreens and other plants are sometimes 

 winterkilled, not so much by the cold as by a sudden spell of 

 warm weather that calls upon stem and leaves for more mois- 

 ture than they can spare at a time when the roots are only 

 feebly absorbing. 



Protection from heat. Since the direct rays of the sun are 

 more harmful than the heated air, tender specimens may be 

 protected in a measure by screens of thin cloth, paper, brush, 

 or lath. Newly transplanted specimens are often sheltered by 

 old newspapers or by a broad leaf, such as that of the burdock 

 or rhubarb. In all such shading it is well to provide for a 

 circulation of air under the cover. Plants in greenhouses and 

 the like are usually shaded by covering the underside of the 

 glass with whitewash. Evergreens and other plants that are 

 not perfectly hardy will often best endure the winter if planted 

 on the north side of buildings or in places where the direct 

 sunshine may be avoided. 



The plants themselves have various devices which protect 

 them from the heat. The leaves of species exposed to the sun 

 are frequently covered with a dense coat of hairs or scales 

 which shade the tender cells. On a hot day the leaves of 

 corn roll up and thus expose a smaller surface for evaporation. 

 The prickly lettuce, the compass plant, and the eucalyptus 

 turn the e^lges of their leaves to the sun, and many tropical 

 species shed part or all of their leaves during the season of 

 greatest heat. The acacias and many other plants of the pea 

 family, with branched leaves, alter the position of their leaflets 



