246 PLANT PHYSIOLOGY 



all growing parts, at least of vascular plants, are in con- 

 stant motion, owing to their growth not being equal on all 

 sides at any one time. The rate of growth changes in the 

 different parts, and because this change in rate is fairly 

 regular and takes place in adjacent parts successively 

 around the plant or part, the motion is regular, circular 

 or elliptical in a horizontal plane, spiral in space. Darwin 

 called it circumnutation. Apparently the regularity of 

 the motion is due to external stimuli rather than to causes 

 inherent in the organism. Experiment* seems to show 

 that the cooperation of the forces to which plants are 

 constantly and successively subjected e. g. gravitation, 

 light, heat, etc. reduces the otherwise wholly irregular 

 movements to order and system, and that without these 

 stimuli these movements produced by unequal growth are 

 entirely irregular. The circumnutation of twining plants is 

 through a somewhat wider orbit than that of other plants, 

 probably because of their greater length in proportion to 

 their thickness and mechanical strength. The tips of the 

 stems and branches do not stand out straight for any 

 great distance ; they tend to droop somewhat. The negative 

 geotropism and the ample nutation of the slender and 

 elongated stems of twining plants cause them to grow 

 spirally upward around suitable supports, t 



Sensitiveness to contact and later their parasitism were 

 probably acquired by dodder and its allies after these plants 

 had developed the twining habit. The nearest relatives of 

 the dodder are still independent twiners, closely resembling 

 the behavior of dodder in its unsensitive periods. Further- 

 more, under stress of insufficient food, the dodder is able to 

 manufacture some food for itself. It will develop chloro- 

 phyll in the usually rudimentary and often otherwise col- 

 ored chromatophores which it contains in large numbers. 



Thigmotropic sensitiveness of other organs and of lower 



* Fritzsche, Curt. Uber die Beeinflussung der Circumnutation durch 

 verschiedene Factoren. Inaug.-Diss. Leipzig, 3 899. 



t For a careful study of the mechanics of twining see Kolkwitz, R. 

 Beitrage zur Mechanik des Windens. Ber. d. D. Bot. Ges., Bd. XIII., 1895. 

 The literature is here cited. 



