[KIRSCH] ORIGIN AND DEVELOPMENT OF RESIN CANALS 49 
On comparing the characters exhibited by the two species in the 
material just described it is easily seen that the shoots of Pinus bank- 
siana, on the whole, showed a much more vigorous growth in the vascu- 
lar cylinder, and more especially in the xylem tissues, than those of 
Pinus strobus. Whether this was due to the fact that the most favora- 
ble geographic limit for the former species was in the vicinity where 
the material was collected, or whether there was some more special 
reason, the writer is not prepared to say, although the former reason 
seems most probable, as the trees of both species from which the shoots 
came were growing within several yards of each other. Whatever be 
the cause for this great difference in the growth of the two species, 
however, the significance of the facts brought out in the above account 
will best be seen as the view of the author regarding the nature of the 
so-called resin passages will be further developed. 
Besides the material mentioned above, the splendid series of slides 
representing the North American Conifere published some years ago 
by Prof. Penhallow, was used freely for comparative purposes and 
proved of great value in confirming certain features presented in the 
original mounts. 
A general view of the material on which the present research is 
based having been obtained, a discussion of the essential features 
which presented themselves in the course of the investigation is now in 
place. 
When the cells of the medullary ray make their appearance in the 
young shoot, they are either isodiametric, or have their major axes in 
the vertical plane. This is well illustrated in Fig. 3, which is a drawing 
from a radial section through the apical region of a shoot of Pinus 
strobus selected from the first lot of material described above. In this 
figure a ray is seen proceeding from the phloem to the protoxylem, the 
vertical character of its component cells being well marked. 
Fig. 4 shows a similar ray taken at the base of the shoot from which 
Fig. 3 was obtained and shows the cells to be still more or less vertical, 
although some of them are beginning to extend and divide in a radial 
direction. These facts bring us to the following considerations in 
regard to the structure of the medullary ray. 
That the cells of the medullary ray exhibit a tendency to grow in 
the vertical direction is evident not only from the above, but from 
certain characters as presented in the mature ray, wherever any of the 
ray elements have had an opportunity for greater freedom of growth 
than other parts of the same ray or than other rays. In this case 
there is not only a decided difference in the radial length of the elements, 
but also a considerable increase in height. This is very well shown 
Section IV., 1911. 4. 
