[KIRSCH] ORIGIN AND DEVELOPMENT OF RESIN CANALS 971, 
Fig. 14 shows à group of parenchyma cells from the same section 
as the above, but in a still more advanced stage, the lumen of the canal 
having taken on its characteristic appearance. 
Fig. 15 is of considerable interest, as it elucidates a seemingly con- 
tradictory appearance. In cross sections, canals are sometimes observed 
that are not in contact with any medullary ray. This is the case in Fig. 
15, but is only a drawing of the same group as represented in Fig. 14 
taken from a section obtained at a slightly lower level of the shoot. It 
is therefore evident that the section was cut at a point below the origin 
of the cells from a ray, and they therefore seem to have arisen indepen- 
dently among the tracheidal elements. 
Figs. 1 and 16 show radial bands of tissue extending from the 
phloem to the protoxylem and are drawn from a section at the base of a 
shoot of Pinus banksiana described in lot one. They both illustrate 
very clearly the formation of the resin canals in the region of the proto- 
xylem. The rays are seen to have started at the point where the vertical 
strands of parenchyma cells arose, and on the formation of the intercel- 
lular spaces some of the medullary ray cells have taken on an epithelial 
character. The latter point is well-illustrated in radial sections of 
Pinus banksiana and Pinus strobus as represented by Figs. 17 and 18 
respectively. These are both taken from second year shoots of the 
respective forms and show how some of the medullary ray elements abut 
on the lumen of the vertical passage, thus functioning as epithelial cells. 
Fig. 19 is a drawing from a shoot of Pinus strobus collected just 
at the beginning of its second year’s growth (May 22, 1907). The canal 
can be seen to take its origin directly from the ray on the right, the 
central row adding to the parenchyma aggregate which is connected 
with the ray on the left by a row of parenchyma cells. Several 
of the ray cells function as epithelial cells, the lumen of the canal 
in this case extending in a tangential direction. The ray on the right 
proceeded from the phloem to the pith, but the central ray commenced 
at the parenchyma aggregation, probably owing its existence to the 
same cause that initiated the parenchymatous strand. 
The medullary rays of the Conifer, as seen in tangential section, 
exhibit two principal forms, as distinguished by the presence or absence 
of a contained resin canal. The horizontal passages in the rays, accord- 
ing to Mayr (19, 185) always take their origin from the vertical ones 
present. Thus, in speaking of the resin cysts of the bast which he 
describes as isolated endings of the obliterated medullary ray canals, 
he says:—“Since the horizontal passages of the Coniferæ never arise 
independently in the medullary ray, but always take their origin from 
vertical passages, it follows that those parts of the plant which possess 
no vertical canals in the wood, of which later, will also lack resin cysts 
