. The anatomy of woody plants. Botany -- Anatomy. foliar traces. As a direct result of this discontinuity of the primary cylinder, the secondary cylinder becomes united only by the gradual broadening of the originally separate secondary segments. In c is portrayed the condition found in the stem of a third rep- resentative of the Lycopsida—namely, a calamite. Here the primary wood is extremely degenerate, and the secondary segments as a consequence are very narrow and quite widely separated. In the herbaceous living Equisetum, the stem of which is depicted on an earlier page, secondary growth
. The anatomy of woody plants. Botany -- Anatomy. foliar traces. As a direct result of this discontinuity of the primary cylinder, the secondary cylinder becomes united only by the gradual broadening of the originally separate secondary segments. In c is portrayed the condition found in the stem of a third rep- resentative of the Lycopsida—namely, a calamite. Here the primary wood is extremely degenerate, and the secondary segments as a consequence are very narrow and quite widely separated. In the herbaceous living Equisetum, the stem of which is depicted on an earlier page, secondary growth has disappeared, being present only as a vestige in conservative organs and regions. The series indicated above illustrates the origin of the herbaceous type by degeneracy. The Lycop- sida have been chosen because they reveal the situation in the clearest manner and with the fewest complications. In the Pteropsida the topog- raphy is rendered less easy of comprehension by the presence of the often numerous foliar gaps which characterize the anatomical organization of the siphonostelic central cylinder of that phylum of vascular plants. The principles involved are, however, the same. It may accord- ingly be stated that discontinuity in the primary siphonostelic central cylinder is due either to gaps related to the exits of traces supplying the appendages or to the local degeneracy of the cylinder itself. These interruptions are perpetuated in the early organiza- tion of the secondary cylinder. If the secondary tissues are also degenerate, as in the existing survivors of cryptogamic groups, a pronounced herbaceous condition is the result. This is, however,. b C FIG. 266.—Diagram showing the effect of degeneracy of the primary wood on the develop- ment of the secondary cylinder. Explanation in the 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 m
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