172 MISSOURI BOTANICAL GARDEN. 
ing of a series of rollers, on the growing branches, and 
causing these bands to press with varying loads upon the 
branch, by attaching weights. For the details of Krabbe’s 
method, the reader is referred to his original paper. 
Krabbe placed different weights on different branches, and 
after having left them for a season, he made microscopic 
examinations of the tissues immediately under the band. 
He attempted by so doing to find what weight, or in other 
words, what radial pressure, would stop the increase in 
diameter of the branch. He holds that this weight would 
be equivalent to the growth energy of the twig.’ 
As a result of his investigations Krabbe concludes that 
the growth energy of the cambium ring and young wood 
cells in coniferous trees is at least 10 atmospheres. For 
hard woods he finds that the growth energy is at least 15 
atmospheres. Krabbe was unable to determine the exact 
value of the expansive force of the limbs and branches with 
which he experimented. His results simply show that 
with the greatest pressures exerted by-his rollers, viz: 10 
atmospheres for conifers and 15 atmospheres for broad- 
leaved trees, growth still took place under the bands. The 
figures which Krabbe obtained are to-day in general accep- 
tance as probably representing the approximate minimum 
growth energy of branches. Friedrich (4) makes the state- 
ment that ‘‘ the increase in diameter of the tree trunk takes 
place with a development of energy equal to at least 10 
atmospheres.”’ 
In order to arrive at some conclusion as to the amount 
of energy required to burst the bands of the bag worm, it 
was necessary to determine, first of all, the pressure which 
the bands were capable of exerting radially. The radial 
pressure exerted by a band placed around a cylinder is de- 
termined by the formula 
pbs io 
y rx oe 
in which P equals the radial pressure exerted by the band 
