Text-book of botany : morphological and physiological / by Julius Sachs ; translated and annotated by Alfred W. Bennett ; assisted by W.T. Thiselton Dyer.

Date:
1875
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Licence: Public Domain Mark

Credit: Text-book of botany : morphological and physiological / by Julius Sachs ; translated and annotated by Alfred W. Bennett ; assisted by W.T. Thiselton Dyer. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    77° Corylus ATeilana, and some other plants were found on the other hand to be hyponastic; when laid horizontally in their natural position they curved upwards, but downwards if reversed, because their hyponasty was stronger than their geotropism. Similar experiments to those made on petioles with respect to heliotropism, showed in many cases the absence of this phenomenon, especially in the case of stolons; and that in other cases it was always positive, but too feeble to overcome the influence of their epinasty. In the case of branches, especially such as are long and slender, more account must be taken of weight in modifying the direction of growth than in that of leaves. The removal of the leaves (e. g. in Corylus) is in this case followed by a sudden curving upward, the result of elasticity ; but this is subsequently intensified by geotropism and in many cases (as in Abies) also by hyponasty. It may be left to the ingenuity of the student to determine the directions of organs by his own observation in any particular case, from the points of view stated above. Sect. 23.—Torsion1. Organs of any considerable length very commonly dis- play torsions about their axis of growth; the striations on the surface of the organ are not parallel to its axis of growth, but run round it in the form of more or less oblique spiral lines, as if the organ were fastened at one end, and then twisted at the other. Torsions of this kind occur in the unicellular internodes of Nitella; they are common in the elongated multicellular internodes of the erect stems of Dicoty- ledons, universal in climbing internodes ; the pedicels of the thecae of Mosses are generally very strongly twisted. Even in flat leaves, as Wichura has shown, torsions of the lamina occur very commonly; they behave like strips of paper fastened at one end and twisted by the other round their median line. These torsions are particularly conspicuous in the leaves of many Grasses, of Allium ursinum, species of Alstroemeria, &c., causing the under side of the lamina to lie uppermost towards the apex2. Since the striae on a twisted organ run spirally round the axis, they must exceed the axis in length; if therefore the torsion is the result of growth, the growth of the outer layers of cylindrical, conical, or prismatic organs (internodes, roots, &c.) must be more rapid or must last longer than that of the inner layers; and in twisted leaves there must be the same difference as respects the growth of the mid-rib in comparison to that of the margins. The fact that at the time of most rapid growth the inner layers generally grow more rapidly than the outer ones (Sect. 13), thus preventing the possibility of torsion, the additional fact that torsion does not gener- ally take place until growth is ceasing, and lastly, the circumstance that etiolated internodes, which in a normal state do not exhibit torsion, usually manifest this phenomenon at the close of their growth, lead to the conclusion that torsion is the result of growth continuing in the outer layers after it has ceased or begun to cease in the inner layers. In twisted leaves, especially those of Alstroemeria, the torsion however begins earlier. If the growth of the outer layers, besides being greater, 1 II. de Vries in the second Heft of the Proceedings of the Wurzburg Botanic Institute 1871, p. 272.—Wichura in Flora 1852, No. 3, and Jahrbuch fiir wissensch. Bot. vol. II, i860.—Braun in Bot. Zeitg. 1870, p. 158. 2 [Similar torsions occur in petals as Cyclamen, fruits as Ailanthus malabarica, and not unfre- quently in pedicels or inferior ovaries as Orchidese, causing the anterior part of the flower to become apparently posterior, and vice versa.—Ed.]