SIGNIFICANCE OF TRANSPIRATION 167 



transpiration than in similar plants growing under conditions of low trans- 

 piration (Freeland, 1937) although there is no actual proportionality between 

 the volume of water absorbed and the quantity of mineral salts which pass 

 into the plant. Since even when plants grow under conditions favoring low 

 transpiration rates they usually obtain an adequate quota of the various min- 

 eral salts, provided they are present in sufficient abundance in the soil, it is 

 difficult to see that this effect is of any very great advantage to the plant. 

 Realization is now fairly general that there is, at the most, only a slight 

 correlation between the rate of transpiration and the rate of absorption of 

 mineral salts from the soil. 



It is also generally considered that transpiration plays an important part 

 in the translocation of mineral salts from the root system to the top of the 

 plant. That some upward translocation of mineral salts occurs in the trans- 

 piration stream is indisputable, but whether this is an important part of the 

 total quantity transported is open to debate. There is considerable evidence 

 that at least a part of the mineral salts which move through the plant are 

 translocated in the phloem (Chap. XXVIII). The rate of transpiration can 

 have little or no direct effect upon the movement of such salts. While high 

 transpiration rates may mean a somewhat higher rate of transport of mineral 

 salts through the plant than low ones it is doubtful if this is of great signifi- 

 cance from the standpoint of the plant. 



3. Supposed Role in the Dissipation of Radiant Energy. — Leaves exposed 

 to direct sunlight absorb large quantities of radiant energy which, unless dis- 

 sipated in some other way, will be converted into heat energy and raise the 

 temperature of the leaf. The possibilities of such an effect are indicated by the 

 following approximate calculations : In direct noon-day summer sunlight the 

 rate of receipt of solar energy is often 1.3 g.-cal. per square centimeter of leaf 

 surface per minute and not uncommonly even greater. The proportion of 

 this actually absorbed varies with the kind of leaf, but will be assumed to be 

 only 50 per cent, which is a conservative estimate. The incident radiant 

 energy which is not absorbed by the leaf is all transmitted or reflected. Such 

 a small proportion of the absorbed energy is used in photosynthesis that it can 

 be neglected in a rough calculation. Hence about 0.65 g.-cal. of energy is 

 absorbed per square centimeter of leaf surface per minute. If the mass of a 

 square centimeter of a leaf is taken as 0.020 g. and its specific heat as 



0.879 g.-cal.^ the rise in temperature per mmute would be -^^ — ^— 



or about 37° C. Since the thermal death point of most plant protoplasm lies 



2 These are the actual values as determined for a sunflower leaf by Brown 

 and Escombe (1905). See also ShuU (1930)- 



