PHYSIOLOGY 249 



exhibited by all growing plants, as their tips do not grow forward in 

 a straight line, but, instead, describe irregular elliptical curves. These 

 movements, which Darwin termed CIRCUMNUTATIONS, while often not 

 perceptible to the eye, are very noticeable in some special organs. 

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

 movement which, in this instance, is induced by the more vigorous 

 growth of the inner side of the young leaves. The same unequal 

 growth manifests itself most noticeably in the leaves of Ferns and 

 many Cycudeae. In the same manner, movements of nutation are 

 caused in other lateral organs when growth is more energetic on either 

 the upper side (epinasty) or on the lower side (hyponasty). From 

 the nutation of the shoots of Ampelopsis quinquefolia a curvature is 

 produced which continuously advances with the increased growth ; so 

 that, by means of its hooked extremity, 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 or of Yucca 

 filamentosa, 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 elon- 

 gated organs must, from purely physical reasons, quickly respond to the 

 effects of unequal growth. The thread-like tendrils of many climbing 

 plants, so long as they are in a state of active growth, afford excellent 

 objects for the observation of nutation movements. If the line of 

 greatest growth advances in a definite direction around the stem, its 

 apex will exhibit similar rotatory movements (revolving nutation). 

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

 climbing plants, and renders possible their peculiar mode of growth. 



The SO-CALLED REVOLVING NUTATION OF TWINING PLANTS is not an 



AUTONOMIC movement, and will be considered later with the 

 paratonic movements. 



Paratonie Movements. — The phenomena of paratonic movements 

 are of the very greatest importance to plant life, for through their 

 operations the organs of plants first assume such positions in the air, or 

 water, or in the earth as are necessary for the performance of their vital 

 functions. A green plant which spread its roots over the surface and 

 unfolded its leaves below ground could not exist, even though all 

 its members possessed the best anatomical structure. The strongest 

 roots would become dried up without the necessary absorption of 

 water, and the leaves could not assimilate in the dark. The organisa- 

 tion and specific functions can have effect only when the root 

 penetrates the soil. Similarly, the leaves are efficient only when 

 exposed to air and light. Seeds are not always so deposited in the 

 soil with the embryonal stem directed upwards and the radicle down- 



