. The anatomy of woody plants. Botany -- Anatomy. 94 THE ANATOMY OF WOODY PLANTS gymnosperms and even in the higher representatives of the group as far up as the conifers. It is in the Gnetales, the most advanced forms among the naked-seeded Spermo- phyta, that vessels first make their appear- ance in the cylinder of the secondary xylem. Fig. 720 represents a smaller ele- ment of this type from the wood of the genus Ephedra. The walls terminating the structure under discussion are dis- tinctly at angles to the lateral ones and are remarkable for the extremely large pits which cover their surfa
. The anatomy of woody plants. Botany -- Anatomy. 94 THE ANATOMY OF WOODY PLANTS gymnosperms and even in the higher representatives of the group as far up as the conifers. It is in the Gnetales, the most advanced forms among the naked-seeded Spermo- phyta, that vessels first make their appear- ance in the cylinder of the secondary xylem. Fig. 720 represents a smaller ele- ment of this type from the wood of the genus Ephedra. The walls terminating the structure under discussion are dis- tinctly at angles to the lateral ones and are remarkable for the extremely large pits which cover their surfaces. The pits of the terminal walls are not only very much larger than the lateral pits, but, with two or three exceptions, they have lost their membranes. In the pores where the membranes still persist a well-marked torus reveals its presence in face view. The pits of the lateral walls of the tra- cheids in the case of Ephedra are not very much narrower than those of the terminal surfaces, but invariably are closed by membranes thickened centrally by a well- marked torus. Fig. J2b shows a some- what larger vascular element from the wood of Ephedra, which has its enlarged terminal pit entirely perforate as a result of the complete disappearance of the mem- branes. In Fig. 73 is shown a vessel in which, as an exceptional condition for the genus under discussion, there is a tendency to fusion on the part of the enlarged termi- nal pit. Although in the Gnetales the phenomena of fusion of the end pits of the vessels is rare, it becomes the rule in the angiosperms, as will be shown at a later stage. At a appears a vessel which is transitional from an element of this type to a. B FIG. 72.—Smaller and larger vessels of Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Jeffrey, Edward C. (Edward Charles),
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