RESINOUS TRACHEIDS 57 



which they closely resemble in many of their reactions, but of a 

 highly refractory and modified character. The same evidence also 

 conclusively shows that the cells in which the plates are devel- 

 oped are normal wood tracheids and not wood parenchyma, which 

 is altogether unknown in any of the Cordaitales.- Any transverse 

 section of the wood of Cordaites will show these resinous plates 

 to be present, usually in much larger numbers than in either 

 Dammara or Araucaria, and they exhibit the same features in dis- 

 tribution (plate 12). Compare also plates 14 and 16. 



We are naturally led to ask, What is the purpose of these resin 

 plates ? The peculiar form in which the resin is deposited and 

 the particular location of the plates points with much force to 

 their connection with some functional activity, since if it were 

 simply a question of the storage of secreted products, 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 Gymnosperms. No exact 

 comparison can be established with other plants, and it is diffi- 

 cult to suggest an adequate explanation. One 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 con- 

 nection 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 development of thyloses in the vessels of the 

 Angiosperms or in the resin passages of the higher Coniferales. 

 It is therefore possible that they may be connected in some way, 

 not at present clear, with a more complete restriction of the 

 circulation to a horizontal direction, and particularly through 

 the medium of the medullary rays as specialized channels for 

 that purpose. Among existing Gymnosperms resinous tracheids 

 are almost exclusively confined to Dammara and Araucaria, 

 though it is a noteworthy fact that similar structures occur rarely 



