386 
AMERICAN JOURNAL OF BOTANY 
[Vol. 9, 
in diameter which could readily be thrust into decayed or succulent stems, 
or between the bark and the wood of firmer stems. These observations 
may so readily be duplicated that no attempt is made to report them 
in their entirety, as a few examples will serve as representative of hundreds 
made on various hosts. They show, in brief, that at night or in the shade 
the temperature of twigs and small branches approximates that of the 
air, whereas in the sunlight their temperature is generally above, some- 
times as much as 20° C. above that of the air. For example, dead twigs 
of the cultivated currant {Ribes sp.) bearing numerous stromata of the 
fungus described by Grossenbacher and Duggar^ as Botryosphaeria rihis 
showed on a clear afternoon the temperatures recorded in table i. 
Table i. Temperature in Degrees Centigrade of Dead Twigs of the Cultivated Currant 
Lying on the Ground, North Carver, Mass., May 26, igzi 
Time P.M. 
Air in Shade 
Bulb of Thermometer 
Under Bark 
Bulb of Thermometer 
in Center of Twig 
1:30 
17 
33 in sun 
27 in sun 
2:00 
17 
36 in sun 
29 in sun 
2:30 
17.2 
37 in sun 
28.8 in sun 
3:00 
17.4 
36 in sun 
27 in sun 
3:30 
18 
18 in shade 
18 * in shade 
4:00 
18.4 
18.4 in shade 
4:30 
18 
17.6 in shade 
* This twig, put back in the sun, rose to 27 in 5 minutes. 
This example is by no means unique, since similar results were obtained 
with currant twigs in other localities, as well as with other dark-colored 
twigs such as those of black cherry {Prunus serotina Ehrh.) and black 
birch {Betula lenta L.). The greatest difference between the temperature 
of the air and that of a currant twig infected with Botryosphaeria rihis 
yet recorded by the writer was on Overlook Mountain, Woodstock, N. Y., 
May 13, 1 92 1, when under continuous observation of fifteen minutes (2:00 
to 2:15 P.M.) such a currant twig showed a temperature of 37° C. while 
the temperature of the air in the shade remained at 15.6° C. As might 
be expected, dark-colored twigs tend to show somewhat higher temperatures 
in direct sunlight than lighter-colored twigs, as may be demonstrated by 
exposing small branches of white, yellow, and black birch side by side. 
That the high temperature of the currant twigs shown in table i was due 
to the heat of the sun is readily shown by the fact that as soon as the 
shadow of the bank on which they were exposed reached them, their tem- 
perature rapidly fell to near that of the air. The same relation is even 
more clearly shown by the record of temperatures of two blackberry [Ruhus 
(probably) allegheniensis Porter] canes given in table 2. These canes were 
near a patch infected with orange leaf rust, so the data given indicate 
^ Grossenbacher, J. G., and Duggar, B. M. A contribution to the life-history, parasit- 
ism, and biology of Botryosphaeria rihis. N. Y. (Geneva) Agr. Exp. Sta. Tech. Bull. 
18. 1911. 
