48] CHAPTER III. SPECIAL PHYSIOLOGY OF MOVEMENT. 211 



These spontaneous changes in position of growing-points are 

 designated generally by the term Nutation. 



All growing members nutate in a more or less marked manner ; 

 but the most conspicuous instances are afforded by slender struc- 

 tures, such as tendrils, and the internodes of twining stems. 



A peculiar form of nutation is commonly exhibited by dorsiventral 

 members, such as leaves. In the early stages the one surface of the leaf 

 grows much faster than the other, thus leading to certain peculiar forms 

 of vernation and aestivation (see p. 43) ; in the later stages the other side 

 grows the faster, and so the expansion of the leaf is brought about. 

 When it is the upper surface which is growing the faster, whether 

 along the transverse or the longitudinal axis of the leaf, it is said to be a 

 case of ejrinasty (Fig. 131) ; when the lower surface, it is said to be a case 

 of hyponasty. A striking example is afforded by leaves having circinate 

 vernation, as many Ferns, Drosera, etc. ; this form of vernation is due to 

 the growth of the leaf being at first longitudinally hyponastic. The 

 convolute, involute, and conduplicate forms are all the result of trans- 

 verse hyponastic growth in the early stages of development of the leaf, 

 whereas the revolute form is the result of transverse epinastic growth. 



48. Induced Movements. All parts of plants which can 

 exhibit movement are also irritable ; that is, they respond to the 

 action of external agents either by a movement or by a change 

 in the rate or the direction of their movement. The following 

 are the principal causes, or stimuli, of movement, or change of 

 movement : 



a. Mechanical ; contact or pressure ; 



b. Variations of temperature ; 



c. Variations in the intensity of light ; 



d. Changes in the direction of incidence of the rays of light ; 



e. Changes of position with regard to the line of action of 



gravity (vertical); 

 /. Differences of degree of moisture in the surrounding medium. 



a. Irritability to Mechanical Stimuli. This form of irritability 

 is most strikingly manifested by motile mature members, and less 

 markedly by certain growing members. 



The following are instances of irritability to contact manifested 

 by mature motile members : by the leaves of the sensitive plants 

 (see p. 174), and by those of Dion sea and Drosera ; by the stamens 

 of Berberis, Mahonia, the Cynarese, and the Cistaceae ; by the lobes 

 of the stigma of Miinulus, Martynia, and Bignonia (p. 176). 



