i6o 



All Food Is Made by Green Plants unit hi 



Fig. 1 86 The barrel cactus 

 of our desert states is a good 

 example of a plant that loses 

 little water in the hot sun. 

 How do you explain this? 



(U. S. DEPARTxMENT OF AGRI- 

 CULTURE) 



5!%''-^j^^- 



imagine the effect of square miles of 

 forest land? You can see, too, the effect 

 that transpiration has on the soil. The 

 small plot of soil in which the birch tree 

 grows would remain moist for a much 

 longer time if there were no tree. An 

 acre of barren soil loses water far more 

 slowly than the same acre planted to 

 grass. 



But plants do not all lose water at the 

 same rate. The cactus in the desert may 

 lose only 0.02 of a quart in a day, in spite 

 of the desert heat which hastens evapora- 

 tion. There are two reasons for this. 

 There is little water in the soil and the 

 cactus usually has a very small surface. 

 Making practical use of this knowledge, 



farmers plant crops like broomcorn with 

 a smaller leaf surface and extensive roots 

 in dry areas. Crops ^\hich lose \\'ater 

 rapidly can be planted in the moist soil 

 of the Eastern and Central states. 



In a drought water is lost faster than 

 it diffuses into the plant; the plant wilts. 

 A cell well-filled with liquid is said to be 

 tiiTfrid. Turgid cells are swollen and firm. 

 If all the cells are turgid the plant is firm 

 or stiff. As water is lost, cells lose their 

 turgidity and the plant wilts. If this loss 

 continues for a long time, the plant dies. 

 Too rapid transpiration is responsible 

 for a great loss in crops. To test your 

 understanding of this paragraph do Ex- 

 KRCISF, 17. 



