BOTANY 



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, in this case of the under side, mani- 

 fests 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 nuta- 

 tion of the shoots of the Wild Vine (Parthenocissus quinquefolia) a curvature is 

 produced which continuously advances with the increased growth. 



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. 



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. 



Besides these nutations which result from growth, autonomic 

 variation movements are also met with, though less commonly. 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 Leguminosae and Oxalideae, also in Marsilia, 

 and are characterised by 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 (cf. Fig. 237). 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 considerably distended by the osmotic 

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

 but also by the persistence of marked tissue-tensions. 



A pulvinus of one of the Leguminosae, such as the Kidney Bean, has the 

 vascular bundle and the sclerenchyma, which are peripherally arranged in the leaf- 

 stalk, united to form a central and easily-bent strand ; this is surrounded by a 

 thick zone of parenchyma (Fig. 277, 3). If from a pulvinus isolated by two 

 transverse sections the middle sheet of tissue is cut out (Fig. 277, 1), the bulging of 

 the cortical parenchyma both above and below shows the considerable tension. On 



