CONSTRICTION OF TWIGS BY THE BAG WoRM. 157 
far the constrictions have been found on the following 
trees: soft maple, sycamore, willow, poplar, sassafras, red 
gum, white oak, locust, apple, cherry, hemlock, bald cypress, 
Virginia pine, Deodar cedar, larch, Juniperus virginiana, 
Juniperus occidentalis, Thuja occidentalis, and Juniperus 
chinensis. It is very probable that similar swellings will be 
found on other species of trees where the bag worm occurs. 
The effect of the girdling action of the band differed 
very materially on different kinds oftrees. Two types may 
be distinguished. In one case the band evidently has 
stopped the passage of plastic materials through the bark 
absolutely. As a result of this an accumulation 
of these substances has taken place on the outer side 
of the band and the cambium layer at this point has 
formed wood cells and bark cells to an enormous de- 
gree, as a result of which the outer part of the twig has 
grown very much in diameter, particularly so immediately 
beyond the band. The portion of the twig towards the 
tree has practically stopped growing entirely; cases of this 
kind are shown on pl. 20 and pl. 24. | 
In the second case the pressure exerted by the band was 
evidently not sufficient to entirely stop the passage of plas- 
tic materials, or only temporarily so. In this case, a slight 
swelling of the portion of the branch outside of the band 
will be formed after a month or so, following the begin- 
ning of wood development. At the end of the first year 
a condition such as is shown on pl. 22, fig. 1 is found. 
This represents a two-year-old maple twig. It will be noted 
that the outer part of the twig is somewhat larger than the 
part towards the trunk, but that there is also a considerable 
increase in diameter of the twig on the side of the band 
towards the trunk; this is shown likewise for the willow, 
oak, red gum, sycamore, etc. (pl. 23). After another 
year’s growth a still further increase in the diameter of the 
outer part of the twig has taken place (pl. 22, fig. 3), but 
the inner portion of the twig, towards the trunk, has like- 
