III. Resin canals in the canadian spruce (Picea canadensis (Mill.) B. S. P.)-An anatomical study, especially in relation to traumatic effects and their bearing on phylogeny
Every student of conifer wood structure is familiar with the importance that in modern anatomical work is attached to the occurrence and distribution of resin canals. It is apparent in classification, identification, and, above all, in discussions of phylogeny. For example, the Pineæ are said to be distinguished by the normal occurrence of resin canals throughout the secondary wood, the Abieteæ by their occurrence practically only in response to wounding. The canals in the latter case are considered by some as being “revived” by injury and by others as in the process of acquirement. However, important though resin canals undoubtedly are, too little is known with regard to them to warrant many of the prevailing conclusions. There has been no systematic study of their origin and distribution, even in a single species. It was to make a beginning towards the filling in of this gap in our knowledge that the present study was undertaken. The spruce was chosen partly because of the abundance of local material, both in nurseries and in the wild state. In fact, although nursery stock was ultimately made use of to perhaps a greater extent than the material from the woods, the study could probably not have been completed on this material alone, so obscure did the principle underlying the distribution of the resin canals seem when viewed merely from a study of nursery plants. There were, however, additional reasons of a scientific nature for the selection of the genus Picea. The resin canals, occurring apparently normally in the wood, are not nearly so abundant as they are in the pines, and are more irregular in distribution. Two low-power photographs have been made to illustrate these points. In Picea sometimes resin canals are completely lacking in an annual ring, whereas in the adjacent area they are fairly abundant (Plate 8, Photo. 1), but not so numerous nor so evenly distributed as they are in the pine, where every ring of the section shows several (Photo. 2); nor is their radial distribution in the year's growth uniform. They occur at various regions in either spring or summer wood, even to the very borders of both, although usually they are more abundant in the earlier part of the season's growth. The distribution of what are ordinarily considered normal .resin canals is thus very sporadic in Picea. Again, in the region of wounds, tangential series quite like those so frequently figured as typical of Abies, are plentiful ( cf . Photos. 23 and 24, Plate 9, the former of Picea, the latter of Abies).