I?(i MNKTKENTH UK TORT. 



leaves, even in the winter season, when insolation values are at their 

 lowest, is tw'o to ten degrees ('. (3.6 to 18 degrees F.) higher than the 

 air temperature, the latter being in bright sunshine. Askenasy^", 

 Ursprung^', Smith'' and Miss Mattliaei' have each found the leaves of 

 plants to be warmer than the air, the ditterence in temperature being- 

 dependent upon the clearness of the air, the season of the year and 

 time of day, the wind velocity and possibly other factors. These results 

 have been obtained notwithstanding the fact that so good an authority 

 as Sachs in his Textbook on Botany states that "The leaves and slender 

 stems of plants cx])oscd to the air are generally colder than the air." 

 To be sure, transpiration is a cooling ])roccss, and Darwin"' has found 

 that a difference of 1.5^ C. may result between the temperature of two 

 leaves, one freely transpiring and the other not at all. But this process 

 cannot overcome the greater heating effect of the absorption of insolation. 

 During the past two years some observation has been made of the 

 temperature of leaves of the garden strawberry, Fragaria vesca, growing 

 on tlie Weather Bureau groiinds at East Lansing, Mich. (lat. 42*^ 44 

 long. 84^ 26' alt. 85-5 ft.). These observations have been made in a 

 rather crude manner, by means of cylindrical-bulb minimum thermome- 

 ters, as shown in Plate V. The growing leaf was simply folded around 

 the bulb and held in close contact with it by means of a pin. Care Was 

 taken to use a new leaf occasionallv, as soon as anv signs of retarded 

 growth or wilting became apparent, due to the somewhat unnatural con- 

 dition of the leaf under observation. It is realized that this method did 

 not give strictly accurate results as to leaf temperature, but the values 

 obtained were sufficiently pronounced to allow for considerable possible 

 error and still show decided differences between the temperature of the 

 plant and that registered by the dry bulb thermometer in the instrument 

 shelter nearby. Readings were made during the greater portion of the 

 growing season thrice daily, 7 a. m., midday, (regularly at 2 p. m. in 

 1916), and at 7 p. m. The minimum temperature as registered by the 

 minimum thermometer wrapped in the plant leaf, were also noted, with 

 the exception of the months of September and October, 1915, when these 

 readings were omitted. In addition readings were made on a "solar 

 radiation" thermometer, consisting of a blackened bulb instrument, in 

 vacuo, during midday, for the season of 1916. Soil temperature read- 

 ings, one inch below the surface were also taken daily in 1916, at the 

 same iiours as those of plant temperature. The exposure of these ther- 

 mometers is also shown in Plate V. The additional plant thermometer 

 shown was used only a portion of the season, serving as a check to 

 determine whether slight differences in adjustment of the leaf about the 

 l)ulb, etc., would cause a difference in the readings. No discrepancies 

 exceeding one degree were noted. The records taken are given in the 

 following tables: 



