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THE AMERICAN NA TURALIST. [Vol. XXXVI II. 



but it is expressed in a different way, and just here we must 

 note a fact of more than ordinary significance. Resin cells are 

 present on the outer face of the summer wood in A. grandis, 

 A. cojicolor, A. ainabilis and A, inagnifica. Groups of resin 

 cells are present in A. nobi/is, A. concolor, A. bract cata and 

 A.firma, but it will be seen that in only one case — A. concolor 

 — are the two forms of distribution presented in the same 

 species. This is in direct conformity with the idea that the 

 resin passage eventually displaces the resin cell, bringing about 

 an obliteration of the latter, and it goes far to support the idea 

 that with respect to these particular structures, the genus Abies 

 occupies a transitional position, standing next to Picea and 

 Pinus, from both of which the resin cells have completely dis- 

 appeared. Furthermore, from another point of view, the grad- 

 ual replacement of the resin cells appears to be indicated by a 

 corresponding reduction in the contained resin. Nowhere is the 

 resin so abundant in the resin cells, as in those genera like 

 Podocarpus and Taxodium, which show no development of resin 

 passages, even in their most simple forms ; but with the develop- 

 ment of resin sacs, as in Abies or Sequoia, or of resin passages- 

 as in Larix and Pseudotsuga, there is a remarkable diminution 

 of the resin, apparently in direct response to its more ready 

 production by more specialized structures. 



The genus Abies then, appears to form a transition group,, 

 having parallelisms with Agathis and Araucaria through the 

 occurrence of resinous tracheids ; with Thuya, Cupressus, etc., 

 through the survival of isolated resin cells approaching oblitera- 

 tion ; with Tsuga, Larix and Pseudotsuga through the develop- 

 ment of rudimcntarx- resin canals leading to. the formation of 

 definite resin passages ; and with Sequoia through the survival 

 "t isolated resin cells and the development of rudimentary resin 

 canals. Through these parallelisms the connection appears to 

 be most direct on the one hand with Sequoia, and on the other 

 with I suga. This relation of Sequoia to Abies has been shown 

 b\ rcnhallow on former occasions (38), and has more recently 

 I'ecn indicated in other ways by Jeffreys {24), but so far as the 

 jMcscnt evidence is of value, it would not permit us to infer that 

 Sequoia, Abies and Tsuga form a continuous and coterminous. 



