222 botanical gazette. [ September, 



One of the trees had a close, smooth bark, and the other 

 a thicker, rough, corrugated bark. There was considerable 

 difference in the diameter of the two trees, the smooth one 

 being about 16 cm., and the rough one about 28 cm. in 

 diameter. To eliminate the influence of this difference in 

 volume as much as possible, the borings were made on the 

 south side of the tree to the depth of 6 cm. Reference to 

 diagram E will show the temperatures of the two trees, a in- 

 dicates the rough barked, and b the smooth barked poplar. 

 On some days there was a difference of 6 to 10 degrees in 

 the temperatures, the smooth barked tree being the warmer. 

 It was noticeable that on cloudy days, as were the 17th and 

 18th, that the variation was not so marked. Although the 

 larger volume of a would have a tendency to lower its tem- 

 perature, yet it hardly seems possible that anything like this 

 difference would be caused on this account. 



Lack of time at present, prevented further experiments 

 being made upon this point, but the above results point, I 

 think, toward the conclusion, even if they do not warrant 

 us in stating positively that the direct absorption of heat is 

 the main cause of the higher temperature of trees, and that 

 it is largely dependent upon the character of bark. 



Botanical Laboratory, Univ. of Wisconsin. 



Explanation of Plate X VIII.— Vertical scale 1° C.=.l inch. Hori- 

 zontal scale, 12 hrs.=.6 inch. A, Showing difference in temperature be- 

 tween evergreen and deciduous trees. B, Showing difference in tem- 

 perature between evergreen and deciduous conifers. (Period from 16th 

 to 19th indicates continuous rainy weather.) C, Influence of denudation 

 of foliage upon temperature of conifers. C 1 , Before stripping off foliage 

 from tree. C 2 , After stripping off foliage from tree; a=tree that was 

 stripped; 6=tree used for control experiment. D, Showing difference in 

 temperature between woods of different densities compared with tem- 

 perature of the air. E, Showing difference in temperature between two 

 trees of the same species, with bark of different nature. 



Paraguay and its flora. I. 



THOM S MORONG. 



Land 



Northern United 



States would say, if they thought of it at all. True enough, 

 for the mercury in the Fahrenheit thermometer hanging in 



