856 
MECHANICS OF GROWTH. 
of Brassica Napüs, 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 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 back- 
wards, becoming finally straight. The same phenomenon occurs in the tendrils of 
CucurhitacecE, which are also at first rolled up inwards, then become straight, and 
are finally rolled up outwards. Other tendrils are at first straight or only slightly 
Fig. 483.— Nutation of the filaments of Dictamnus Fraxinella; the filaments of the stamens whose anthers have not 
yet opened are bent downwards ; those with anthers already burst are bent upwards. 
concave 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 TropcBolum fnajus, Diciamnus Fraxinella (Fig. 485), Parnassia palustris'^, 
&c., and in long styles Hke 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 another ^ Most lateral shoots behave in the same manner as ordinary leaves, 
growing at first only quickly enough on the outer side to become appressed to the 
primary axis in vernation, afterwards more rapidly on the inner side, by which they 
become straight and diverge at a greater or smaller 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, 1869.] 
^ Vide infra under Fertilisation, Chap. VI. 
