Resin ducts are common in the Pinaceae. The comparative anatomy of stems and leaves of 50 species and two varieties from ten genera has been investigated. The structure and distribution of resin ducts differ among genera. Resin ducts occur in foliage leaves of ten genera of Pinaceae. Cortical resin ducts are absent in the stems of Pseudolarix and Larix. Resin ducts only occur in the secondary xylem of stems of Pinus, Picea, Cathaya, Larix, Pseudotsuga and some Keteleeria species. All of the epithelial and sheath cells are alive and thin-walled in the resin ducts of stem cortex and mesophyll. Except for Pinus the epithelial cells of resin ducts in the secondary xylem of stems have thick, lignified walls. Comparative study shows there are obvious differences in the resin ducts of different genera; apparent differences do not exist, however, in the resin ducts of different species of the same genus. According to the structure and distribution of the resin ducts in ten genera of Pinaceae, a synoptical arrangement of the genera is given and generic relationships of the Pinaceae are discussed.
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