Abstract
IV.—Special Types in Special Environments FROM the previous papers it will be clear that degree of subordination to the stem accounts in large measure for the exteht to which leaves vary from the primitive ovate-lanceolate type. Where they are still so most subordinated, there will be a strong tendency towards the long pointed ribbon-like form, and also a marked inclination towards decurrence. This combination of peculiarities is well seen in several thistles, and in comfrey, as also to a less extent in many epilobes and stellarias. Compare Ver6ascum thaftsus, and other mulleins. From these extreme cases, in which leaf and stem are not fully differentiated from one another, one can trace several gradations, through square stems with sessile leaves (as in certain St. John's worts) up to merely sessile stem- leaves, or leaves that clasp the stem with pointed or rounded auricles. Wherever lines exist along the stem, they may be observed in pairs up to the point where a leaf is given off, and they are undoubtedly surviving marks of the primitive unity of stem and leaf. The same may be said of rows of hairs, like those of S/diana media and of Veronica charneedrys. There can be little doubt that special selective causes (protection against creeping;nsects, &c.) have often come into play in preserving or modifying such decurrent wings, stem-lines, auricles, clasping stipules, and rows of hairs; but as a whole they nevertheless point back distinctly to the origin of dicotyledonous stems from superposition of leaves and midribs upon one another. They are rudimentary forms of stemlamina.
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ALLEN, G. The Shapes of Leaves 1 . Nature 27, 511–514 (1883). https://doi.org/10.1038/027511b0
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DOI: https://doi.org/10.1038/027511b0