298 BOTANY pakt i 



number of growth curvatures or nutations are associated with this 

 growth, and it might almost be said that there is hardly such a thing 

 as growth in a straight line. The tips of the organs describe extra- 

 ordinarily irregular curves in space ; they exhibit " circumnutations," 

 as was discovered by Darwin. While these curvatures are usually so 

 slight as not to be perceptible without the aid of special methods, cases 

 exist in which organs exhibit very conspicuous, striking, and regular 

 autonomic growth curvatures. 



The unfolding of most leaf and flower buds, for example, is a nutation move- 

 ment which is induced by the more vigorous growth of the upper side of the young 

 leaves (epinasty). The same unequal growth manifests itself most noticeably in 

 the unrolled leaves of Ferns and many Cycadeae (hyponasty). The stems of many 

 seedlings are, on their emergence from the seeds, strongly curved, and this aids 

 them in breaking through the soil. By the nutation of the shoots of Quinaria a 

 curvature is produced which continuously advances with the increased growth ; so 

 that, by means of its hooked exti'cmity, a shoot is better enabled to seek out and 

 cling to a support. 



When the unequal growth is not confined to one side, but occurs alternately on 

 different sides of an organ, the nutations which result seem even more remarkable. 

 Such movements are particularly apparent in the flower-stalk of an Onion, which, 

 although finally erect, in a half-grown state often curves over so that its tip touches 

 the ground. This extreme curvature is not, however, of long duration, and the 

 flower-stalk soon becomes erect again and bends in another direction. Thin and 

 greatly elongated organs [e.g. tendrils) must, from ])urely physical reasons, quickly 

 respond to the ell'ucts of unequal growth. 



If the line of greatest growth advances in a definite direction around the 

 stem, the apex of the latter will exhibit similar rotatory movements (revolving 

 nutation). This form of nutation is characteristic of the tendrils and shoots of 

 climbing plants, and facilitates their coming in contact with a support. The so- 

 called llEVOLVING NUTATION UF TWINING rLANT.S is UOt, llOWeVer, au AUTONOMIC 



MOVEMENT, aud will be considered later with the paratonic movements (p. 312). 



Besides these nutations which result from growth autonomic 

 variati(jn movements are also met with, though less commonl3\ They 

 are almost confined to foliage leaves, and indeed to those which have 

 pulvini at the base of the petiole and of its further ramifications. 

 Pulvini occur especially in Legumino.sae and Oxalideae, also in Marsilra, 

 and are characterised b}^ a structure which fits with their particular 

 function. 



In the ordinary parenchymatous cell the cell-wall, owing to its 

 growth in thickness, ceases to be stretched ; on plasmolysis it therefore 

 does not in full-grown cells contract in the same degree as it does in 

 growing cells (p. 256, Fig. 177). Conversely on an increase of the in- 

 ternal pressure the wall only becomes slightly stretched. In some 

 cases, however, and the pulvinus is an example, the cell-walls even 

 in their fully-grown state are consideral)ly distended by the osmotic 

 pressure. This is shown not only by their behaviour on plasmolysis, 

 but also by the persistence of marked tissue-tensions. 



