CONSTRICTION OF TWIGS BY THE BAG WORM. 159 
of these one is shown on plate 23, fig. 2. In every case 
the outer part of the branch was dead, and no growth had 
taken place in the twig between the constriction and the 
tree. The marked inability of the conifers to adapt them- 
selves to these unusual circumstances is very striking. 
STRUCTURAL CHANGES CAUSED BY THE BANDS. 
The changes which take place in the branch of a tree as 
aresult of the pressure exerted by the bands of the bag 
worm are of two distinct types, one of which may be 
illustrated by the arbor vitae and the other by the maple 
or sycamore. Inthe arbor vitae and other conifers, growth 
starts under the band in June following the year in which 
the band is attached to the tree. Where the expanding 
force of the tree is great enough to burst the band, or 
where the band is weak, no change takes place which dif- 
fers in any way from the normal development of the par- 
ticular twig. Where the band happens to be a strong one 
which cannot be burst by the growing twig, changes of a 
very radical nature make their appearance a month or more 
after the beginning of the growth period of the twig. A 
longitudinal section made through such a 
twig after a year’s growth of the arbor 
vitae is shown in fig. 1. It will be noted 
that the cambium layer formed a number 
of rows of wood cells immediately under 
the band, but that after a very brief 
period the formation of wood in the 
twig, beginning at the band and below it, 
stopped entirely, while the part towards 
the outside of the band continued to grow 
vigorously, producing the peculiar swell- 
ing above referred to. The most striking change in the 
immediate vicinity of ‘the band consists in the enormous 
development of bark. Immediately above and below the 
constriction the bark is two or three times as thick as it is 
normally (fig. 1). The wood cells formed immediately 
1. ARBOR VITAE. 
