CONSTRICTION OF TWIGS BY THE BAG WORM. 171 
streets of our cities; the lifting and pushing power of roots 
and trunks is furthermore frequently noted in the forest, 
where large boulders are pushed aside or lifted. Another 
familiar instance of the energy exerted by the growth of 
plant tissue is found in the roots of most higher plants, 
which exert a certain amount of energy in pushing aside 
soil particles. Although this growth energy may some- 
times be very large, it frequently is not sufficient to over- 
come the obstacles to which growing parts are subjected. 
This is frequently illustrated by roots which grow in the 
clefts of large boulders— they are frequently unable to 
burst these, and as a result, are forced into a flattened form. 
Many climbers exert a sufficient pressure upon the trunks 
around which they are growing to arrest development. 
Wires and other bandages frequently bring about girdling 
of trees very similar to those described for the bag worm. 
While it has been realized that the amount of energy 
exerted by growing tissues may sometimes be very large, 
very little is as yet known as to how large the amount of 
energy developed by growing tissue actually is. Inthe course ~ 
of the investigation on the constrictions caused by the bag 
worm bands, it was suggested that these bands might serve 
to indicate something as to the extent of the growth energy 
of young twigs. In the largest per cent of the cases the 
twigs burst the bands; they must therefore exert a radial 
pressure outward greater than that usually exerted by the 
band in an opposite direction. A series of experiments 
was therefore undertaken to determine how great the pres- 
sures were which the bands exerted upon the twigs, with 
the object of using the results as a guide to determine the 
growth energy of the twig. 
Of the investigations hitherto made to determine the ex- 
tent of the growth energy of plant tissues, there is practi- 
cally only one which needs consideration. Krabbe (6) 
undertook some years ago to determine the extent of the 
growth energy of various trees, by placing bands, consist- 
