56 THE ROYAL SOCIETY OF CANADA 
contact with it, but passed between wood elements. The parenchyma 
aggregate thus formed a projection into the tracheidal tissue at this 
point in the section, several other such projections appearing in the 
region of the cambial ring. 
Fig. 11 is drawn from the same slide as Fig. 10 and shows the com- 
mencement of a medullary ray just previous to the formation of a verti- 
cal group of parenchyma cells. This tends to prove that the same 
factors which call forth the initiation of the ray at this period were also 
instrumental in originating the vertical strand of cells. The latter, on 
the continued growth of the tissues, would afford a place for the forma- 
tion of a resin passage. 
Fig. 12 affords a view of the growing region of a vigorous terminal 
shoot of Pinus banksiana in which a second series of resin canals was 
being formed, about twenty-five rows of wood cells having been laid 
down. A medullary ray is seen passing from the bast to the wood, 
parenchyma cells enclosing it on either side below the cambial region, 
and thus affording a starting point for two intercellular passages separ- 
ated by a ray, such as were frequently encountered in the specimen from 
which this was drawn. 
The groups of parenchyma cells thus laid down through the agency 
of the medullary rays, during periods of active growth accompanied 
by a superabundance of food in the region where the rays are situated, 
do not, as a rule, extend as rapidly in a radial and tangential direction 
as the tracheids bordering on them. Since, moreover, the walls of the 
parenchyma elements are very thin, conditions are favorable to the 
formation of schizogenous spaces, as the following shows. Newcombe 
(24, 404) in discussing cavity formation, states that, “As far as observa- 
tion has extended, all cavity formation during primary growth is in its 
beginning schizogenous . . . . . . The cause for the appearance of 
these schizogenous clefts is to be found in the rapid extension of the 
peripheral zones of tissue opposed to the more slowly extending or 
wholly non-extending tissue in the localities in which the clefts arise.” 
In the material under discussion the greatest extension of the tissues 
surrounding the parenchyma aggregates was sometimes in a tangential, 
sometimes in a radial direction. If in the former a slit with its major 
axis in a radial direction, if in the latter one extending in a tangential 
direction would result, the former being usually the case, as is illustrated 
in Fig. 13. This drawing shows a group of parenchyma cells in contact 
with a medullary ray and was obtained from the same section as Fig. 
12, but several rows of cells distant fromthe cambium. The initiation of 
the intercellular passage is clearly seen. 
