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THE AMERICAX XA TCRALIST. [Vol. XXXVIIL 



the relations are somewhat complex, evidence does appear of 

 such a nature as to suggest their derivation. In Finns inopSy 

 P. torreyana, P. pnngens, P. clansa, P. tceda, P. palnstris and 

 P. cnbensis, we frequently find thick-walled parenchyma cells 

 and characteristic ray tracheids coterminous with one another. 

 This does not mean a simple association, since in nearly all such 

 cases, as typically presented by P. palnstris, they also show a 

 graduated structure of such a nature as to confirm the belief that 

 the one passes into the other by structural gradations. That 

 such is the case cannot be doubted, and if further confirmation 

 were needed it is afforded by the precisely parallel relations to 

 be met with in the formation of resin cells and resin canals. A 

 further fact of much significance from the standpoint of devel- 

 opment, is that such interchangeable relations are peculiar to the 

 highest types of the genus Pinus. But we may ask, what is 

 the function of these structures which make their appearance 

 only in the higher Coniferae ; what is the proper significance of 

 their appearance there, and do any other plants offer parallel 

 examples } 



In the so-called medullary rays of Lepidodendron selaginoides 

 (49» P- 141) there are numerous reticulated or spiral elements 

 which are undoubtedly of the nature of tracheids, and they may 

 be held to represent the ancestral form of the ray tracheids in the 

 Coniferae, toward which they bear the same relation that exists 

 between the spiral protoxylem element and the characteristic 

 wood tracheid with bordered pits. From this it is apparent that 

 the ray tracheid of Pinus or Tsuga represents a primitive struc- 

 ture which reappears in response to conditions of growth and 

 structural alterations of such a nature as to demand the interpo- 

 sition of more simple, because more primitive, elements for the 

 proper performance of necessary functional activities. These 

 activities, in the case of Lepidodendron, are probably expressed 

 in the radial distribution of water (5 1, p. 141), and we are no 

 doubt correct in assuming similar activities to be carried on in 

 the higher Coniferae. In all those species which present the 

 primitive structure of the thin-walled ray cells, both fossil and 

 recent, there are no tracheids to be found. As a tendency to 

 thickening of the wall arises, there is also developed a sporadic 



