CONSTRICTION OF TWIGS BY THE BAG WORM. 133. 
The results obtained were calculated according to the 
formula given above; and, following Krabbe’s method, the 
final figures were calculated to show thepressures in grams 
per square millimeter of bark surface, which were then re- 
duced to atmospheres of pressure. For this purpose 10 
grams were used as equivalent to 1 atmosphere of pres- 
sure per square millimeter. In the case of the maple some 
400 bands were tested. It is proposed to test a great 
many more bands in the future in order to have a larger 
number of figures to work with. 
Before drawing any conclusions as to the results of the 
strength tests, it should be stated that the use of the bands 
as an indication of the amount of growth energy exerted 
by the twig must be taken with some caution. No claim 
is made that the results to be discussed are in any way 
absolute; the bands are an extremely variable quantity ; 
they differ in width and thickness, and the error in meas- 
urement is consequently to be considered. It was further- 
more suggested that there might be a very decided differ- 
ence in the strength of the bands, depending upon whether 
they were wet or dry, and likewise upon their age. The 
former point was considered of great importance, and 
accordingly a large number of bands were broken after 
they had been soaked in water for five days. It was 
thought possible that as a result of wetting during rains in 
the early summer, the bands might be considerably weak- 
ened, and that it would be at such a time that they were 
burst by the twigs. The results obtained from the tests 
with soaked bands in no way differed, however, from those 
obtained with dry bands, and it is accordingly the belief 
that the soaking practically does not weaken the bands at 
all, or at least very little. Concerning the effect of age, 
most of the bands were taken and tested during May and 
June, 7. e., after they had been exposed to the weathering 
influences during the fall and winter. It is therefore be- 
