- andit is difficult to suggest an ad 
62 | BOTANICAL GAZETTE [yULy 
tube. Sometimes there is enough resin to fill the tracheid for a 
considerable portion of its length, at others only enough to produce 
very delicate plates, while in the case of a cell with a wide lumen the 
resin may run down along one side or collect in masses without 
connection across the cavity. It is not unlikely that the presence 
of gas bubbles in the cells at this stage may play a part in forming 
the plates and in determining their location, since in some instances 
a thin plate may be found at a 
considerable distance from the 
‘ray and without-visible connec- 
tion with it. 
What is the significance of 
these plates? PENHALLOW says 
as follows: 
The peculiar form in which the resin 
is deposited and the particular location 
of the plates point with much force to 
their connection with some functional 
activity, since if it were simply a ques- 
tion of the storage of secreted prod- 
ucts, the latter would hardly be 
disposed as found, but rather after the 
manner common to so many of the 
Cupressineae; and this suggestion gains 
strength from the fact that with respect 
to the peculiar form of the resin masses 
as well as their location in the tissue, 
the Cordaitales are peculiar among the 
ymnosperms. No exact comparison 
can be established with other plants, 
4 juate expl i ne thing does seem clear, 
however, and that is that since these plates are of an impervious nature and 
developed in some cases, at least, in connection with a special constriction of the 
tracheid cavity, they offer and possibly are specially designed to afford a definite 
obstruction to circulation in a vertical direction. In this sense they may be 
designed to serve the same general purpose that is accomplished by the develop- 
ment of tyloses in the vessels of the angiosperms or in the resin passages of the 
higher Coniferales. It is possible, therefore, that they may be connected in 
some way, not at present clear, with a more complete restriction of the circula- 
tion to a horizontal direction, and particularly through the medium of the 
medullary rays as specialized channels for that purpose. Among existing 
RP 
Fic. 1.—Transverse section of Pinus 
albicaulis. 
