856 MECHANICS OF GROWTH. 



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 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 

 CucurbitacecB, 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. 485.— Nutation of the filaments oi Dictamntis 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 TropcEolum majus, Diciamnus Fraxinella (Fig. 485), Parnassia palustris ^j 

 &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 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. 



