742 PART IV. THE PHYSIOLOGY OF PLANTS. 



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, 

 Buch 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. 60) ; in the later stages the other side grows the faster, and 

 so the expansion of the leaf is brought about. When it is the ventral sur- 

 face which is growing the faster, whether along the transverse or the longi- 

 tudinal axis of the leaf, it is said to be a case of cpinasty (Fig. 479) ; when the 

 dorsal 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 hypo- 

 nastie. The convolute, involute, and conduplicate forms are all the result of 

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

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



12. Irritability (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 ; 

 Z>. 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. 



Irritability to contact is manifested by the following mature 

 motile members : by the leaves of the sensitive plants (see p. 689), 

 and by those of Dionsea and Drosera ; by the stamens of Berberis, 

 Mahonia, the Cynareea, and the Cistaceee; by tho lobes of the 



