402 ROYAL SOCIETY OF CANADA 
the separation and consequent disorganisation giving rise to the inter- 
cellular canal. Hence Thome is wrong when he states that the proto- 
xylem elements become functionless on the completion of the secondary 
xylem, and tear apart, for, when the secondary xylem is already formed 
the protoxylem, if present, whether functionless or not, remains intact. 
This is clearly proven in regions of slow growth, and follows logically, 
for at that time the cause of the separation at an earlier stage is non- 
existent, the secondary xylem having ceased its growth. 
When the the protoxylem is fully formed in regions of very active 
growth, the secondary xylem is still in a highly formative state, and on 
the separation and disorganisation of the former, cannot very well assume 
the conductive functions which it carries on later. Hence, at this time, 
the only efficient vehicle for the conduction of the transpiration stream 
is the canal, which thus assumes this most important function at a cri- 
tical period in the growth of the plant. The canal must therefore be 
regarded as a continuous vessel which functions alone until the secondary 
xylem is able to assume the duties assigned to it in the division of labor. 
That the canal is well suited for this purpose is easily seen, for the ir- 
regular character of its wall and the protoxylem remains in its cavity 
would aid greatly in supporting the water-column, whilst in size it 
greatly exceeds the area of the disorganised protoxylem. Where the 
growth is slow and the protoxylem is consequently not torn apart, no 
canal is formed, the protoxylem retaining the conductive functions. 
It would not be at all unreasonable to assume that the intercellular 
passages in the vascular bundles function in the above manner wherever 
they occur, and it therefore remains to be seen why thyloses do not ap- 
peur normally in all the canals, both in the ferns and in the higher plants, 
or, in other words why the canals become functionless in some whilst 
they retain their function in others. 
When the sap is conducted only by the intercellular canal in Pteris 
aquilina, the pressure on the parenchyma cells which form its wall must 
be enormous, and must certainly produce some physiological effect on 
them. Now, the xylem plate in the vascular bundles of Pteris and other 
Filices occupies a very large portion of the bundle, and so when the 
xylem elements become fully formed and assume their proper function, 
the force of the stream ascending by means of the canals must become 
greatly lessened, for the stream which before possessed only one means 
of ascent is now proceeding along many channels. In this way the pres- 
sure exerted on the living cells which form the wall of the canal is greatly 
reduced. These wood parenchyma cells are, however, very active ani 
highly sensitive to stimulation, and when the pressure is thus suddenly 
diminished, they begin to extend into the cavity of the canal. 

