How Substances are Transported and Removed 201 



to express transpiration in standard units. For greenhouse 

 plants, which have been the most carefully studied from this 

 point of view, it has been found that while the transpiration in 

 one hour from one square meter (roughly a square yard) of leaf 

 ranges according to circumstances all the way from near nothing 

 up almost to 300 grams (11 ounces), the generalized average, or 

 conventional constant, is 50 grams per square meter (nearly 

 2 ounces per square yard) per hour, i. e., 50 gm'-h, by day and \ of 

 this quantity, 10 gm-h, by night, which equals 30 grams per square 

 meter, 30 gmVi (an ounce per square yard) per hour, day and 

 night together. Upon this basis, an average leaf during an ordi- 

 nary summer season transpires an amount of water equal to its 

 own area, and a centimeter (| of an inch) deep. These quantities 

 are well worth remembering. 



The first sensation of the student as he really comprehends 

 these data, especially whenever they are yielded by experiments 

 of his own, is always one of surprise at the largeness of the quan- 

 tity. It is, indeed, this copiousness of transpiration, rather than 

 the existence of the process, which is the remarkable thing about 

 it; and it helps to explain a number of more or less familiar 

 phenomena. Thus, the rapidity with which leaves alwaj s wilt 

 when cut from their stems, and the quickness and completeness 

 with which plants can dry out the soil of their pots, are conse- 

 quences of transpiration. In this way some plants can serve as 

 good drainers of marshy soils. Thus Eucalyptus trees, especially 

 active transpirers, have been used for this purpose in the Roman 

 Campagna with such success that the marshes have become 

 freed from the former scourge of malaria-carrying mosquitoes, 

 and therefore habitable by man; while the malaria-repelling 

 virtue often ascribed in this country to Sunflowers, which are 

 sometimes planted around dwellings with this end in view, has 

 the same genuine scientific basis. It is also transpiration condi- 

 tions chiefly that determine which kinds of plants can be grown 

 in dwellings as house plants. House plants are by no means the 



