TRANSPIRATION 



83 



mist soon accumulates on the inside of the glass. In time 

 drops of water form. The experiment may be varied as 

 shown in Fig. 116. Or invert the fruit-jar over an entire 

 plant, as shown in Fig. 117, taking 

 care to cover the soil with oiled 

 paper or rubber cloth to prevent 

 evaporation from the soil. Even 

 in winter moisture is given off by 

 leafless twigs. Cut a twig, seal the 

 severed end with wax, and allow 

 the twig to lie several days: it 

 shrivels. There must he some up- 

 ward movement of water even in 

 winter, else plants would shrivel 

 and die. 



175. When the roots fail to sup- 

 ply to the plant sufficient water to 

 equalize that transpired by the 

 leaves, the plant wilts. Transpiration from the leaves and 

 delicate shoots is increased by all of the conditions which 

 would increase evaporation, such as higher temperature, dry 

 air or wind. The breathing pores are so constructed that 

 they open and close with the varying conditions of the 

 atmosphere, and thereby regulate transpiration. However, 

 during periods of drought or of very hot weather, and 

 especially during a hot wind, the closing of these stomates 

 cannot sufficiently prevent evaporation. The roots may be 

 very active and yet fail to absorb sufficient moisture to 

 equalize that given off by the leaves. The plant wilts. 

 Any injury to the roots or even chilling them (149) may 

 cause the plant to wilt. On a hot, dry day note how the 

 leaves of corn "roll" towards afternoon. Early the fol- 

 lowing morning note how fresh and vigorous the same 

 leaves appear. Water is also forced up by root-pressure 

 (146). Some of the dew on the grass in the morning 



To illustrate transpiratioji. 



