RESIN PASSAGES 



If, then, we ask what value such structures have for taxo- 

 nomic purposes, we find them to be of well-defined importance. 

 It has already appeared that in Tsuga the occurrence of resin 

 cysts is of well-defined value for specific differentiation, and the 

 same rule is also applicable to Sequoia sempervirens and to four 

 species of Abies. In the higher Abietineae, inclusive of Pseu- 

 dotsuga, Larix, Picea, and Pinus, the invariable association of 

 resin passages in the wood and in the medullary rays not only 

 serves to separate these genera from all those in which resin cysts 

 only may occur, but it also differentiates them absolutely from 

 all the remaining genera. Such association, therefore, constitutes 

 a feature of great value. More particularly, the thin-walled 

 epithelium of Pinus at once separates that genus from the other 

 three, which are invariably characterized by thick-walled epithe- 

 lium. Such generic differentiations are greatly emphasized by 

 the occurrence of thyloses. These are typically developed in 

 Pinus, where they are always thin-walled and almost invariably 

 present. They are, therefore, of definite value as supplementing 

 other features previously described. In the other genera, how- 

 ever, their presence in either the cyst or the resin passage, 

 where they are generally thick-walled, is of so sporadic a nature 

 as to give them no definite value, and we therefore find that for 

 specific diagnoses such structures may be neglected. 



We are now in a position to present an answer to the ques- 

 tion, How are the resin passages related to the phylogeny of 

 the Coniferales? In order to present an intelligent answer to 

 this question, it will be necessary to recall the facts already 

 discussed in connection with the resin cells, and bring them 

 into relation with our discussion of the resin passages. 



In the genus Sequoia it has been shown that the general 

 course of development of the resin cells is essentially the same 

 as in Cupressus, etc., while it has also been shown that the genus 

 presents in other respects a somewhat remarkable deviation. 

 Of the two existing species both show the distribution of the 

 resin cells to be of the typically primitive form, i.e. scattering. 

 Nevertheless there are also in Sequoia sempervirens definitely 



