[Chap. XXV TRANSPIRATION AFFECTS PLANT DEVELOPMENT 241 



problem. In the case of a green alga or a flowering plant growing sub- 

 merged in water transpiration certainly is not an essential process. If the 

 plant is removed from the water and placed in the air, transpiration 

 occurs only as an unavoidable process and its effects soon cause death. 

 Transpiration goes on whether its effects are harmful or beneficial, just 

 as water evaporates from a wet towel or any other saturated object 

 exposed to the air. Consequently if transpiration is an essential process 

 in plants, it must be limited to those in which some part or the whole 

 plant is exposed to the atmosphere. 



One should be clear whether the term "essential" is applied to the 

 survival of the plant, or merely to some particular quantitative difference 

 in its development. We have already seen that leaves are thicker, have 

 a thicker cuticle, more layers of palisade cells, and a greater amount 

 of woody cells when they develop in dry air where transpiration is 

 high than when they develop in moist air where transpiration is low. 

 Transpiration mav be essential to many such quantitative differences 

 among plants, without being essential to the survival of the plant. 

 These differences are merely the ultimate effects of transpiration on 

 development. 



Among the claims that transpiration is an essential process are: (1) 

 that it is necessary to cool the plant on hot summer days, (2) that it is 

 necessary in the absorption and conduction of salts from the soil to the 

 leaves, (3) that it increases the water supply of the plant, (4) that it 

 regulates the water content of the plant, ( 5 ) that it maintains the water 

 supply of leaf cells, and (6) that it would not occur in plants if it were 

 not of some value to them. 



The fallacies implied in the last four statements should be so obvious 

 from data discussed in previous chapters as to make further discussion 

 unnecessary here. Some additional data may help to evaluate the first 

 two statements. 



The cooling effect of transpiration is most frequently cited as its 

 essential feature. It is claimed that if transpiration did not occur, the 

 temperature of leaves in full sunshine on a hot summer day would rise to 

 150° F. or more; this rise is sufficient to kill protoplasm. The well-known 

 fact that the evaporation of water lowers the temperature of the mass 

 from which it is evaporating is sufficient evidence that transpiration has 

 a cooling effect on the plant, but it is not sufficient evidence that it is 

 essential, or that it prevents the temperature of a leaf from rising to 150° 

 F. every day in summer. 



