242 TEXTBOOK OF BOTANY 



When one compares the enormous differences in the rate of transpira- 

 tion in the date palm, coconut palm, and cactus in the tropics he might 

 expect them to differ greatly in their internal temperature. Yet measure- 

 ments have shown that the temperature of all plants corresponds fairly 

 closely to that of the atmosphere in which they are growing. The daily 

 evaporation of water from the columnar cactus cited earlier in this chap- 

 ter would lower its temperature less than 1/1000 of a degree, yet it 

 daily absorbs sufficient radiant energy from the sun to raise the tempera- 

 ture and kill the protoplasm if there were no other way by which heat 

 passed from the plant to the surrounding air. A hot stove loses heat by 

 radiation and convection. A plant warmer than the surrounding air also 

 loses heat by radiation and convection. 



Measurements have shown that when the temperature of a plant is 

 more than a few degrees above that of the surrounding air the loss of 

 heat from the plant by radiation and convection usually exceeds the 

 amount lost by transpiration. The greater the difference between the 

 temperature of the plant and that of the surrounding air, the less impor- 

 tant is the loss of heat b)^^anspiration in comparison to that lost by 

 radiation and convection. 



The relation of transpiration to salt absorption and conduction is an 

 indirect one. The date palm mentioned earlier absorbs and loses 40,000 

 times as much water as the cactus plant cited in the same table. The roots 

 of the coconut palm grow in both brackish and fresh water. If the 

 absorption of salts depended upon the absorption and transpiration of 

 water, one would expect to find corresponding differences in the salt con- 

 tent of these plants. No such differences occur. Salts enter the roots of 

 plants as molecules and ions by diffusion. This diffusion of salts is 

 undoubtedly influenced by electrical charges, but, as we have already 

 seen, it is independent of the diffusion of water into the cells. After the 

 salts have entered the plant, further diffusion accompanied by flow in 

 the xylem vessels and protoplasmic streaming accounts for their move- 

 ment within the plant. 



Transpiration may indirectly influence the total amount of salts ab- 

 sorbed by plants through its effects upon several other processes. Two 

 plots of bean plants were placed under experimental conditions so 

 that the plants in one plot grew in a saturated atmosphere, the others in 

 an atmosphere having only 25 per cent humidity. Transpiration was 

 much greater in the plants exposed to the drier atmosphere. These plants 

 were not as tall as those that grew in the saturated atmosphere. Their 



