. A manual of the North American gymnosperms [microform] : exclusive of the cycadales but together with certain exotic species. Bois; Trees; Gymnosperms; Gymnospermes; Arbres; Wood. RKSIN PASSAGES '43 If, then, we ask what value such structures have for taxo- nom,c Hrposes. wc find them to be of well-defined importance It has already appeared that in Tsuga the cKcurrencc of resin cyst, .8 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 Abietinea;. inclusive of Pseu dotsuga, Larix, Picca
. A manual of the North American gymnosperms [microform] : exclusive of the cycadales but together with certain exotic species. Bois; Trees; Gymnosperms; Gymnospermes; Arbres; Wood. RKSIN PASSAGES '43 If, then, we ask what value such structures have for taxo- nom,c Hrposes. wc find them to be of well-defined importance It has already appeared that in Tsuga the cKcurrencc of resin cyst, .8 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 Abietinea;. inclusive of Pseu dotsuga, Larix, Picca, and Pinus, the invariable association of resm 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, constitute, a feature of great value. More particularly, the thin-wallcd epithelium of Pinus at once separates that genus from the other three, which are invariably characterized by thick-walled epithe- hum. Such generic differentiations arc greatly emphasized by the occurrence of thyloses. These are typically developed in I'inus. where they are always thin-walled and almost inNariably 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 In order to present an intelligent answer to this question, it will be necessary to recall the facts already discussed m connection with the resin cells, and bring them mto relation with our discussion of the
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