1889.] BOTANICAL GAZETTE. 221 



that of the oak was 12.5 . The temperature of the oak re- 

 mained about the same as the air temperature, while the pop- 

 lar was often 5 higher during the heat of the day- 



The question naturally arises, what causes the difference 

 in temperature between these trees? Does it exist in the 

 different densities of the two woods, or can it be assigned to 

 some other cause? There are several sources to which we 

 might look to find the cause. The different densities of the 

 woods, the heat evolved by chemical changes in the tree, 

 the color and texture of the bark, causing different degrees 

 of absorption, might be suggested as possible causes. It is 

 more than likely that more than one of these possible causes 

 are involved. 



If we compare the temperature of the larch and oak, we 

 find that although quite a difference exists between their spe- 

 cific gravities, but little difference is found in their tempera- 

 tures. The difference in their specific gravities is .1234 or 

 about 20 per cent., while the difference in the mean daily 

 temperature for the length of the experiment does not exceed 

 |° C. This evidently does not furnish a satisfactory reason 

 for the difference in temperature. The difference in the ab- 

 sorptive powers of different colors is well known, but here I 

 think it plays but a small part. The poplar used for the ex- 

 periment was covered with a smooth greenish-gray bark, 

 while in the oak the bark was roughened and darker gray in 

 color. The darker the color, the greater the amount of heat 

 absorbed, would lead one to think that the temperature ot the 

 oak ought to be higher than the poplar, but such was not the 

 case, so that we must seek still farther for a cause. 



Heat due to chemical changes, if sensible to the thermom- 

 eter, might produce some difference, as the metabolism ot 

 the poplar was probably greater at this time than in the oak. 

 But from the experiment conducted upon living and dead 

 timber with the apple tree, it is hardly possible that this 

 would account for the considerable difference that existed 

 between these two trees. The influence ot the bark upon 

 absorption plays, I think, a far more important part than 

 either of the above. It was seen that the oak and the larch 

 differed but little in respect to temperature, and here we find 

 quite a similarity in the texture and character ot the Daric. 

 Both are of a neutral grav tint and are more or less rough- 

 ened, although scarcely corrugated. To test the influence 

 of the bark upon the temperature, two trees were taken ot 

 the same species (Populus grandidentata), but with bark ot 

 different character. 



