332 BOTANICAL GAZETTE | NOVEMBER 
symmetrical. A part of the primary cortex, and with it the 
resin canals, may be cut off bythe cork cambium. This may be 
only local, or include only a part of the resin canal. The part 
of the canal thus disturbed is soon filled by the ingrowing 
epithelial cells. It is evident that, with the intrusion of the 
epithelial cells, the resin is pressed into the undisturbed part of 
the canal, causing this part to distend and increase in size, by the 
radial and tangential division of its epithelial lining (fig. 70, 4 
and c). In this way resin vesicles are formed, often two or more 
in each canal. Other factors ,besides the disturbances due to 
the cork cambium may cause tylosis; thus great temperature 
variations of exposed and shaded sides of the stem, and with it 
variations in the transpiration of the bark. Tylosis begins in the 
resin canal as soon as the loss of water from the cells and cell 
walls reaches a certain per cent. With the filling up of the 
resin canal the radiai and tangential division of the epithelial 
cells of the resin vesicle still continues, and with it the vesicle 
increases in size, receiving at the same time all or nearly all of 
the resin of the original canal. Not only do the resin vesicles 
contain all the resin of the canal, but also the resin which is all 
the time being secreted by the tissues of the primary and second- 
ary cortex. The great quantity of resin secreted by A. balsamea 
tends also to form the vesicles, as they begin to form even in 
four-year old internodes, and before tylosis has begun. With 
the increase in age of the bark the vesicles increase in size, so that 
in trees 20 to 100 years old the trunk of the tree always contains 
the balsam vesicles in great numbers. The larger ones are 
always found on the lower part of the tree trunk. They can be 
_ seen on the outside, where they appear as swellings or blisters 
onthe bark. The periderm and outer part of the primary cortex 
give way to the pressure of the vesicle as it increases in size by 
the radial and tangential division of its lining cells. With the 
increase in size of the vesicle it becomes surrounded by several 
layers of cells resulting from the division of the epithelial | 
ing cells. These become thick walled and soon break away 
from the surrounding tissues of the cortex, so that older vesicles 
