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
Catalogue details

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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    If another side then grows more rapidly, it becomes convex, and the curvature changes its direction. Curvatures of this kind caused by the unequal growth of different sides of an organ may be called Nidations. They occur most commonly and evidently when growth is very rapid, and consequently in organs of consider- able length, and are produced under the influence of a high temperature either in darkness or when the amount of light is very small. When two opposite sides of an organ grow alternately more and less rapidly, curvatures are caused first on one side and then on the other ; it will, for example, bend first to the left, then become erect, and then bend to the right side; as occurs, e.g. in the long flower-scapes of Allium Porum, which finally take an erect position when their growth is ended. It is much more common for the apices of erect stems above the curved growing part to move round in a circle or ellipse, the region of most active growth moving gradually, as it were, round the axis. This kind of nutation may be termed a Revolving Nidation. Since the apex of the stem is constantly rising higher during the nutation owing to the elongation of the part below it, its revolving motion does not take place in a plane, but describes an ascending spiral line. This form of nutation occurs in many flower-stalks before the unfolding of the flowers, as in those of Brassica Napus, where the movement ceases when growth is completed, and the stem finally becomes erect. It is very general in climbing stems and in almost all erect stems that bear tendrils; but bilateral tendrils also revolve at the time when they are about to take hold of a support1. In bilateral appendicular organs nutation does not usually take the form of a revolving motion, or only to a subordinate extent, as in tendrils. The outer or dorsal side more often grows more rapidly so that the organ is curved concavely to the primary axis, and the inner side afterwards begins to grow more quickly, so that the organ finally becomes straight, or even concave on the dorsal side. This is the case in all strongly developed foliage-leaves, very strikingly in those of Ferns, which are at first rolled up towards the axis, and then unroll, often bending over backwards, becoming finally straight. The same phenomenon occurs in the tendrils of Cucur- bitaceae, which are also at first rolled up inwards, then become straight, and are finally rolled backwards. Other tendrils are at first straight or only slightly con- cave inwards like leaves in vernation, but are afterwards rolled backwards. Move- ments of nutation are very common and easily observed in stamens with long fila- ments, as Tropceolum majus, Diclamnus Fraxinella (Fig. 454), Parnassia palustris2, &c., and in long styles like those of Nigella saliva, &c. They occur at the time of the maturity of the sexual organs, and serve to place the stigmas and anthers in the positions adapted for the conveyance of pollen by insects from one flower to another3. Most lateral shoots behave in the same manner as ordinary leaves, grow- ing at first only quickly enough on the outer side to become adpressed to the primary axis in vernation, afterwards more rapidly on the inner side, by which they become straight and diverge at a greater angle from the primary shoot. 1 See Sect. 25, On the Twining of Tendrils. 2 [On the stamens of Parnassia, where there is not properly any movement of nutation, see Gris, Compi rend, Nov. 2, 1868; and A. W. Bennett, Journ. Linn. Soc. vol. XI, p. 24, i86j.—Ed.] 3 Vide infra under Fertilisation, Chap. VI.