CONSTRICTION OF TWIGS BY THE BAG WoRM. 161 
does in the arbor vite, but it occurs on both sides of the 
band. Asa result of the growth both 
above and below the band, two lips or 
rings of tissue are formed (fig.2—4). 
which at first are separated, but 
which soon meet above the band and 
become closely appressed (fig 3). At 
the end of the first year the two lips 
have usually joined, and a continu- 
ous ring of wood fiber has once more 
been formed. As soon as the union 
has taken place between the two lips 3, SOFT MAPLE. 
the wood fibers once more assume their normal longi- 
tudinal position (fig. 4). 
The parts immediately above 
and below the band show an abund- 
ant supply of starch throughout 
the summer and late into the fall, 
_ a from which it is evident that the 
FE] flow of nutrient substances from 
the outer part of the twig to the 
lower part is never entirely stopped 
by the constriction. 
The wood cells formed under 
the band during the process of 
healing differ considerably from 
the normal wood tissue. The first wood cells formed 
under the band in June, following the year when the 
band is first attached to the twig, are normal in every 
respect. Although the band is fastened firmly around 
the twig, it is probable that it at first exerts very little 
pressure upon the growing cambium cells; it is probable 
likewise, that the band stretches to some extent during 
the first month or two following the beginning of the 
growth period of the twig. At a certain point, how- 
ever, a time is reached where the band no longer stretches, 
11 
MAPLE. 
