530 TRANSFORMATION OF ENERGY 



by observations in seedlings (i.e. Zea mais; Fig. 163). The nutations performed 

 by many of the higher plants are still more complicated and more irregular if 

 several zones of growth come into operation at the same time. The movements 

 of the inflorescence of Yucca, for example, are very remarkable ; indeed during 

 the process of unfolding the inflorescence looks as if it were pathological, but by 

 and by it assumes its normal straight character. Fig. 164 shows these changes in 

 position. Curvatures also arise in the cellular filaments of the Zygnemaceae, 

 due to the irregularities of growth which are always taking place and which 

 cause the filaments to assume different positions. Fig. 165 shows a filament of 

 Spirogyra drawn at short intervals (compare E. WINKLER, 1902). 



It is impossible for us to enumerate all the cases of autonomous nutation re- 

 ferred to in the literature on the subject ; we must confine ourselves in what 

 follows to examples of especially uniform nutations. 



The best known are the revolving nutations (NOLL, 1885 ; WORTMANN, 

 1887), where the oblique or horizontal apices of plants move forward in circles 

 or ellipses. This type of nutation corresponds exactly to that shown by Desmo- 

 dium, but it arises from the fact that one flank grows more vigorously than the 

 other and that this increment of growth affects new longitudinal areas in regular 

 succession. Circumnutation may be compared also with the outwardly similar 

 movements of the apices of twining plants, and here, as there, we find a twisting 

 of the apex of the shoot on its long axis in order to avoid torsions. The differ- 

 ence between the two sets of phenomena lies in this, however, that whereas in 

 the case of twining, gravity plays an active part, we have here to do with purely 

 autonomous movements. Such movements occur very prominently in tendrils 

 and are obviously of the utmost importance as aiding them in finding a support. 

 Circumnutatory movements occur also in seedlings, stola, &c., and, in these 

 cases, when the curve is a much elongated ellipse, they approach the pendulum 

 movements in character, as may be especially well seen inAllium scorodoprasum. 

 Careful observation certainly shows that in the case of pendulum movements the 

 oscillations are by no means always in the same plane, any more than, in cir- 

 cumnutation, are true circles or ellipses described by the growing points. Hence 

 it would be difficult to separate these two types of nutation from each other. 



The same is true of a further distinction which has been drawn between 

 periodic and transitory nutations. Typical transitory or solitary nutations occur 

 in many organs which are curved at an early stage in their growth and after- 

 wards become straight. The cotyledons, the hypocotyl. and the root of the 

 embryo frequently exhibit special curvatures characteristic of the type, and 

 which appear to be entirely autonomous. With these may be associated the 

 curvature manifested by embryonic organs in the bud. Stamens, floral and 

 foliage leaves are very frequently curved inwards owing to excessive growth on 

 the under side (hyponasty), and become straightened later on, owing to increased 

 growth on the upper side (epinasty). The bud form is thus due to hyponasty 

 of the lateral appendages. Not infrequently it happens that the epinastic out- 

 curvature is not completed in one movement, but that a more vigorous de- 

 pression alternates with a feebler erection, thus constituting a periodic nutation. 

 Transitions between transitory and periodic nutations occur in those cases where 

 epinastic growth, so to speak, shoots beyond the mark, as, for example, when the 

 leaflets of Aesculus, converging upwards in the bud position, bend downwards on 

 the opening of the bud and then, during unfolding, owing to renewed hyponastic 

 growth, spread out approximately horizontally. Epinasty and hyponasty co- 

 operate with diageotropism and diaheliotropism in bringing about the definite 

 rest position of dorsiventral organs. External factors often aid in bringing 

 about this result, or they may operate antagonistically to it. It is impossible 

 for us to discuss the action of epi- and hypo-nasty in greater detail ; the matter 

 becomes especially complicated, inasmuch as in addition to autonomous move- 

 ments induced nastic movements also come into play (e.g. photonasty). 



