162 MISSOURI BOTANICAL GARDEN. 
and from then on the band exerts an increasing amount 
of pressure upon the underlying cambium cells. A word 
should be said at this point as to the structure of the 
band. It is composed of a series of fine strands of silk 
which are densely interwoven, forming a very compact 
mass; the individual fibers appear to be pasted together 
with a cementing material; they are closely adapted to the 
irregularities of the bark, so that the pressure which the 
band as a whole exerts is a very even and regular one. 
After the pressure has begun to act a marked change in 
the character of the wood cells formed under the band is 
noticeable, which gradually becomes more marked as the 
amount of wood formed below the band increases. After 
about six weeks all further formation of wood tissue under 
the band ceases. On plate 25 are shown sections of nor- 
mal maple wood (fig. 1) and of a portion of the wood 
formed under a band in a maple twig (fig. 2); likewise 
sections of wood of the red gum from normal (fig. 3) and 
constricted regions (fig. 4); and plate 26, figs. 1 and 2, 
shows two photomicrographs of a cross section of maple 
and red gum twigs immediately under the band. It will 
be noted that as the pressure increases the number of 
vessels decreases proportionately, and at a period when the 
growth has almost ceased, the formation of vessels has 
almost stopped. Under the increased pressure the wood 
cells take on an entirely different appearance; instead of 
the comparatively thin-walled, large-lumened tracheids, 
one finds thick-walled, very small-lumened tracheids form- 
ing, which continue to be formed up to the very time when 
all growth ceases. The vessels which form at the period 
of greatest pressure show a tendency to become round and 
thick-walled, as wellas smallerin diameter. The medullary 
rays show a decided tendency to buckling. Instead of 
growing out in a straight line radially, they bend towards 
one side or the other as if pulled laterally. This bending 
is more striking in some instances than others. It is of 
Tene st ee 
Yate 
