CONSTRICTION OF TWIGS BY THE BAG WORM. 167 
way influence the division of the mother cells of the cam- 
bium, but that any change which takes place occurs after 
the wood cells have been formed. The cambial cells 
divide even under great pressure, though when the pressure 
reaches a certain point it is probable that the rate of divi- 
sion is considerably reduced. The formation of new cells 
evidently stops abruptly; in other words, the conditions 
for the development of new cells from the cambium are 
such that the latter is able to form these new cells irre- 
spective of the growing pressure up to a certain point, and 
when this point has been reached growth stops. The cam- 
bium appears to be more or less uninfluenced in the char- 
acter of its cell divisions by pressure exerted upon it, and 
is able to continue its activity up to a maximum point. 
- Concerning the discrepancy between the findings of 
Kiister and those of Krabbe and myself, I am inclined to 
ascribe the formation of the parenchymatous tissue in the 
branches investigated by Kiister to the unequal pressure to 
which the apposed surfaces were exposed. I have found 
atissue which resembles that described by Kiister under 
similar conditions in stems of maple and other trees in the 
region immediately below twining vines, particularly where 
the latter caused a constriction in the bark of the trunk 
around which they were growing. 
The change in direction of growth of the cambial cells 
resulting in a shifting of the axis of the wood cells to an 
angle of 90 degrees from that usually occupied by them, 
has already been referred to above. 
Summing up the changes which occur in the region of 
wood under the band, it may be said that with an increase 
of pressure smaller and smaller cells are formed, because of 
the inability of the newly formed wood cells to expand to 
a normal size; the cambium continues forming wood cells 
of the usual kind but at a decreased rate as the pressure 
increases. Where the pressure is reduced, at the edges of 
the band, a shifting of the cambial cells takes place, result- 
