266 



LEAVES 



transpiration, the plant is not in danger. But it is not uncommon 

 to see Corn with leaves rolled and Potatoes, Cotton, Clover, and 

 other plants wilted during dry hot days. These plants are losing 

 water faster than it can be replaced from the roots. These 

 plants are in danger because their living cells are becoming dry, 

 and too much drying results in death. More plants die on 

 account of transpiration than anything else. 



The important thing for the plant is the maintenance of a 

 proper balance between supply and loss of water. The plant can 

 endure rapid transpiration, if a copious supply of water is coming 

 up from the roots; but, if the ground is dry about the roots, the 

 root system small, or water hard to obtain from the soil, as is the 



Fig. 238. — A portion of a cross 

 section through a node of Sugar 

 Cane, showing rods of wax secreted 

 by the epidermis. Enlarged many- 

 times. After De Bary. 



Fig. 239. — A portion of a sec- 

 tion through a Mullein leaf, show- 

 ing the epidermis with its branched 

 hairs. 



case in soils that are cold or frozen, then even a small amount of 

 transpiration may be injurious. 



Protection against Injuries Resulting from Transpiration. — 

 Plants may be protected against the injurious effects of trans- 

 piration by having their transpiring surface modified, or by 

 having the soil moisture increased or conserved. 



There are various ways in which plants modify their transpiring 

 surface. Some plants, such as the Carnation, Pine, and many 

 plants of the desert, have the epidermis of their leaves covered 

 with a heavy layer of cutin. Sometimes, as in Cabbage, Sugar 

 Cane, and Wheat, the epidermis is covered with a waxy bloom. 

 (Fig. 238.) Many plants are protected by a covering of hairs. 

 {Fig. 239.) Some plants, such as the Cacti of the desert, have 

 reduced their leaves to mere spines which offer only little trans- 



