766 MECHANICAL LAWS OF GROWTH. 



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 Nutations. 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 fiow^er-scapes oi 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 Nutatioji. 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, wher^ 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 support \ 



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- 

 bitaceas, 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 afterw-ards rolled backwards. Move- 

 ments of nutation are very common and easily observed in stamens with long fila- 

 ments, as TropcBolimi majus^ Dictamuiis Fraxinella (Fig. 454), Parnassia palustris^, 

 &c., and in long styles like those of Nigella saliva, Sec. 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 

 another ^ 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. 



* See Sect. 25, On the Twining of Tendrils. 



^ [On the stamens of Parnassia, where there is not properly any movement of nutation, see Gris, 

 Comp. rend, Nov. 2, 1868; and A. W. Bennett, Journ. Linn. Soc. vol. XI, p. 24, 186^. — Ed.] 

 ^ Vide infra under Fertilisation, Chap. VL 



