78 THE ROYAL SOCIETY OF CANADA 
would arise, when the abnormally small amount of food presented to 
the growing tissues would deprive the fusiform ray of many of its ele- 
ments and thus cause the canal to taper off and end blindly. That this 
is the manner in which the passage ends is seen from what Jeffrey states 
further on (12, 326). “Although the horizontal canals always start 
from a vertical series, they by no means always end in the next outward 
vertical series, even when one is present. More trequently they end 
blindly, as in the one last described above.” 
In regard to Pseudolarix, Jeffrey (14, 15) noted that resin passages 
were absent in the cortex ofthe vegetative branches and the root. In 
their place, large mucilage cells of about the same calibre as the resin 
passages of the cortex in other parts, made their appearance. This 
brings us to a consideration of the mechanics of schizogenous cavity 
formation. 
Schizogenous spaces arise owing to the fact that certain areas of a 
tissue do not keep pace in growth with surrounding areas, and hence 
differences of tension arise. In such a case something has to give way, 
and in the case of the resin cells, the thin cellulose walls of the cential 
cells are parted from each other and give rise to a cavity which increases 
in size as the growth of the surrounding cells increases. This forma- 
tion can be prevented in several ways: 
1. When some cell or cells within the zone where a canal would 
normally be formed has the power of increasing in circumference pro- 
portionately to the extension of the cells which surround it. This is the 
case here for the large mucilage cells are of this character. (See Figs. 
26, 27, 28 and 31 of 14.) This is probably also the reason why no canals 
are present in the woody Dicotyledons as a whole, for the extension of 
the vessels would provide for the necessary release of tensions. 
2. Where the cells in the normal position of the schizogenous 
cavity have the power of dividing proportionately to the extension of 
the surrounding tissues. This is no doubt the case where large aggre- 
gates of parenchyma are present without containing a resin canal, as 
noted above in Sequoia. 
3. Where all the eomponent cells of a tissue have a uniform rate of 
growth. This may be the case in the cortex of Tsuga. 
In Cedrus, in addition to the vertical ducts, horizontal ones also 
-arise. The absence of the horizontal canals in the medullary rays of 
the other forms may be explained by the conditions presented in the 
growth in length of the shoot. Thus, Anderson, in describing the witch 
broom of Abies balsamea (2, 311) says: “The diseased annual shoot 
is shorter, but has a greater diameter than the normal.” This, accord- 
ing to our view of the origin of the medullary ray canals would suffice 
to explain their absence here, for there would be even less tensions in 
