272 BOTANY 



2. Growth Curvatures 



Movements from which curvatures result are, for the most part, 

 produced by the unequal growth of living organs. The unequal 

 growth is due, partly to internal causes which are still undetermined, 

 and partly to the operation of external influences. The movements 

 resulting in the first case are spontaneous, and are called AUTONOMIC 

 MOVEMENTS or NUTATIONS ; in the second case the movements are 

 the result of external stimuli, and are distinguished as IRRITABLE or 

 PARATONIC (AITIONOMIC) MOVEMENTS. 



Autonomie Growth Curvatures (Nutations) are most plainly 

 apparent in young actively-growing organs, although nutations have 

 been shown to be 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 CIRCUM- 

 NUTATIONS, while often not perceptible to the eye, are very noticeable 

 is some special organs. 



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

 ment 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 Cycadeae. In the same manner, movements of 

 nutation are caused in lateral axes when growth is more energetic on either the 

 upper side (EPINASTY) or on the lower side (HYPONASTY). Epinastic curvatures are 

 often greatly increased when poisonous gases are present in the air, and may also be 

 started by the organ's own weight. This is termed " load-curvature " by WIKSNER. 

 The stems of many seedlings are, on their emergence from the seeds, strongly 

 curved. By the nutation of the shoots of Ampclopsis 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 elongated organs (e.g. tendrils) 

 must, from purely physical reasons, quickly respond to the effects of unequal 

 growth, 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 facilitates their coming in contact with a support. The so-called REVOLVING 

 NUTATION OF TWINING PLANTS is not, however, an AUTONOMIC MOVEMENT, and 

 will be considered later with the paratonic movements. 



Paratonic (Aitionomie) Growth Curvatures. The paratonic move- 

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

 their operations the organs of plants first assume such positions in 



