RESIN PASSAGES 151 



the former view, most especially as all anatomical data confirm 

 the relative positions of the genera as determined by the devel- 

 opment of the resin passage. But assuming for the moment 

 that the latter view is the correct one, let us see where it would 

 lead us. It would, first of all, necessitate a direct reversal of the 

 structural sequence, and this in turn would impose the necessity 

 of placing the genus Pinus at the bottom of the scale, while 

 those genera, like Taxus, Torreya, Dammara, etc., which have 

 no resin cells even, would be at the top. I venture to suggest 

 that such a proposition would meet with instant opposition, even 

 from the advocates of the idea that the resin passage has pre- 

 ceded the other forms of resin-bearing structures. The whole 

 question appears to turn upon our recognition of what consti- 

 tutes the most impressionable portions of the stem structure, 

 and therefore the regions within which structural changes are 

 initiated. In this connection the evidence of both paleobotany 

 and recent botany brings out certain facts with great force and 

 throws them into strong relief. They are as follows : 



1. The mucilage canals of the Eusporangiate ferns may be 

 regarded as the ancestral forms of the resin passages among the 

 higher plants, but they are obviously the successors of, as they 

 are derived from aggregates of, mucilage sacs as simple paren- 

 chyma cells. 



2. Resin passages are wholly unknown in the wood of the 

 stem of ferns, the Cycadofilices, the Cycads, Cordaites, or 

 Araucarioxylon. 



3. Resin cells are known and are abundant in the pith and 

 bark of Cordaites, but they are absent from its wood. 



4. Resin passages are known in the bark and in the pith of the 

 Cycadacese, of Dammara and Araucaria, and of the Coniferales in 

 general. They also occur in the wood of the peduncles of Sequoia 

 and in the xylem of the first year's growth of vigorous shoots in 

 Sequoia and Abies. They likewise occur in the leaves generally. 



5. In Sequoia Burgessii, from the Eocene, resin passages 

 occur in the medullary rays, but they do not traverse the wood 

 longitudinally, though isolated resin cells do occur there. 



