WHY PLANTS WILT 



81 



as higher temperature, dry air or wind. In especially hot 

 weather, when the wind is brisk and the air dry, the roots 

 may be very active and yet fail to absorb sufficient moisture 

 to equalize that given off by the leaves. Any injury to the 

 roots or even chilling them (156) 

 may cause the plant to wilt. On a 

 hot, dry day, note how the leaves 

 of corn "roll" toward afternoon. 

 Early the following morning, note 

 how fresh and vigorous the same 

 leaves appear. Water is also forced 

 up by root-pressure (154). Some of 

 the dew on the grass in the morn- 

 ing may be the water forced up by 

 the roots; some of it is the condensed 

 vapor of the air. 



169. The wilting of a plant is due 

 to the loss of water from the cells. 

 The cell walls are soft, and they col- 

 lapse. A toy balloon will not stand alone until it is inflated 

 with air or liquid. In the woody parts of the plant the cell 

 walls may be stiff enough to support themselves, even though 

 the cell is empty. Measure the contraction due to wilting 

 and drying by tracing a fresh leaf, and then tracing the 

 same leaf after it has been dried between papers. The 

 softer the leaf, the greater will be the contraction. 



Review. — What relation do root-hairs have to soil-particles? What 

 is the effect of the chilling of roots? Of what use to the plant is water'.' 

 What essential elements are taken from the soil? How many elements 

 are essential for the plant? What is the ash? What elements does the 

 fanner add as fertilizers? How may the nitrogen supply of the soil be 

 increased? What plants possess the root nodules? What is soil 

 inoculation? What is transpiration? When does a plant wilt? 



129. To illustrate transpiration. 



