2 MOVEMENT 



cells, owing to the presence of special locomotory organs, cilia, or flagellae, 

 are able to swim about actively in water. 



Among plants it is only in the case of small organisms that active 

 locomotion is possible, and frequently only during a particular stage of the 

 life history. Since the response due to a stimulus is always dependent upon 

 the character of the resulting movements, a freely motile plant may travel 

 towards a source of illumination, whereas a rooted plant responds in a less 

 degree by growing and curving towards the illuminated side. In spite of 

 this difference, the actual perception and stimulation may be identical in the 

 two cases. 



The movements of free-swimming plants appear to have a more pur- 

 poseful nature, simply because they resemble the movements of animals. 

 As a matter of fact the power of perceiving and responding to stimuli is 

 equally developed in plants rooted to the soil. Free-swimming plants, it is 

 true, lend themselves more readily to experimental studies, because they 

 usually react more rapidly than plants which can respond only by a change 

 in the rate or character of growth. Since most plants fall in the latter 

 class, and since curvatures are usually produced by growth, we shall confine 

 ourselves at first mainly to movements of this character. 



The fact that in large plants the power of growth and movement are 

 not strikingly evident has caused plants to be popularly regarded as ' still 

 life.' Hence the rapid movements of Mimosa pudica were regarded as 

 extraordinary for a plant, and the same applies to the spontaneous 

 movements performed by the lateral leaflets of Hedysarum gyrans ] . If 

 mankind from youth upwards were accustomed to view nature under 

 a magnification of 100 to 1,000 times, or to perceive the activities of weeks 

 or months performed in a minute, as is possible by the aid of a kine- 

 matograph, this erroneous idea would be entirely dispelled 2 . 



Movements serve a variety of aims and purposes, and need to be con- 

 sidered not only as regards the causes which produce them and the way in 

 which they are carried out, but also as regards their importance to the 

 plant. We are, however, less concerned with oecological explanations 

 than with the determination of causes and mechanism. 



In every case a response to a stimulus indicates a specific irritability, 

 although the nature of the response will vary in different plants according 

 to their nature and properties. We can, however, distinguish between 

 autonomic, autogenic, or spontaneous stimuli on the one hand, and 

 aitiogenic, induced or paratonic stimuli on the other, and the same 

 applies to the movements resulting from internal or external stimula- 



1 Pfeffer, Die Reizbarkeit d. Pflanzen, 1893, p. 9. (Reprint from Verb. d. Ges. deutscher 

 Naturforscher u. Aerzte, 1893.) 



2 Pfeffer, Jahrb. f. wiss. Bot., 1900, Bd. xxxv, p. 738. 



