58 



MISC. PUBLICATION 257, U. S. DEPT. OF AGRICULTURE 



Table 26. — Average dry weight^ and ash content of 1,000 white pine seedlings under 



different light intensities 



Light condition 



Dry weight 



Total ash 



Ash content 



No shade . - . ... . 



Grams 

 0.063 

 .034 

 .010 



Grams 

 0.005 

 .003 



.001 



Percent 

 8 29 



Half shade - ... 



9 35 



Full shade ._. 





10 20 









It is thus seen that the plants in no shade transpired 21 times as 

 much water but accumulated only 5 times as much ash as those in full 

 shade. The amount of ash, therefore, increases with the transpiration 

 but not directly, indicating that the minerals are not absorbed pas- 

 sively along with the water but are regulated in their entrance by 

 forces Yvithin the plant. 



Whatever may be the effect of transpiration on the ash content, 

 most workers are agreed that transpiration helps to lower the leaf tem- 

 perature, which otherwise, in full sunlight, might get dangerously high. 

 Thus Copeland (88), to cite a recent publication, found that the leaves 

 of transpiring chaparral plants in California showed a temperature of 

 4° to 10° C. lower than the surrounding air. The temperature was 

 lowered more when the under surface of the leaf was exposed, permit- 

 ting free transpiration, than when the upper surface was exposed. 

 Furthermore, species which transpired less rapidly showed less cooling 

 effect. 5 



As to whether or not transpiration is a genuine function, it is the 

 opinion of the writer that although a certain percentage of transpira- 

 tion is possibly a necessary evil, transpiration does help the plant in 

 that it cools the leaf mechanism and that it aids (even if it is not 

 absolutely necessary for) the diffusion of mineral substances through- 

 out the plant. Herbaceous plants might perhaps get along without 

 the aid of the transpiration stream through root pressure, diffusion, 

 capillarity, osmotic pressure, and other physical forces acting within 

 the plant; but to distribute efficiently to the tops of tall trees the 

 necessary supply of mineral salts is a somewhat different matter. 

 Diffusion processes are not very rapid under such circumstances, and 

 the bulk of evidence points to the fact that for a normal development 

 of plant organs, as well as for the transport of minerals, the transpira- 

 tion stream is of great importance. 



SUMMARY AND SUGGESTIONS FOR RESEARCH 



While some water is lost under abnormal or exceptional conditions 

 by bleeding and guttation, most of the water is lost through the 

 stomata of the leaves during the process of transpiration. The number 

 and location of the stomata influence this loss and there are also 

 specific and individual differences among trees, but the chief variations 

 in losses are associated with environmental factors, such as light, 

 temperature, humidity, soil moisture, etc. 



5 Many other papers written from a purely physiological point of view and dealing mostly with herbaceous 

 forms tend to show the cooling effect of transpiration. 



