168 MISSOURI BOTANICAL GARDEN. 
ing in the turning of the long axis of the wood fibres at an 
angle of 90 degrees to their usual position. 
It has been stated that as soon as the pressure exerted by 
the band becomes great enough to retard the expansion of 
newly formed wood cells, an increased growth becomes evi- 
dent above the band in conifers, and both above and below 
the band in hard woods. The cambial tissue in the region 
not acted upon by the pressure at first continues to grow 
normally, but very soon increases its rate of cell division. 
Under the changed condition of tension a change of direc- 
tion of the cambial layer takes place ; instead of remaining 
a sheet parallel to the long axis of the branch, the cam- 
bium both above and below the band is curved outward. 
The wood and bark cells produced, because of this 
altered position of the cambium, instead of growing in a 
radial direction, as they do normally, are turned so that 
their axes gradually approach a direction at an angle of 90 
degrees from their normal position. With this changed 
position in the direction of growth of the wood and bark 
cells comes an increased development of both, so that after 
awhile the branch is very much thicker immediately above 
and below the band than it is at some distance from it. 
The swellings appear as the lips already referred to, 
The tissue composing the two lips is, to all intents and 
purposes, wood tissue. The tracheids are normal in char- 
acter, although many of them are somewhat shorter than 
those formed under normal conditions; there is, in other 
words, no development of what might be termed callous 
tissue or anything resembling the tissue known as * wound 
wood.’’ The development of the lips, in many respects, 
however, resembles the development of callous tissue after 
a ring of bark has been removed from a twig. Inthe latter 
case, the stimulus, as a result of which an increased devel- 
opment of tissue takes place at the margins of the cut, 
consists in the interruption of the strains normally active in 
the growing region of the twig. By cutting away a certain 
