MOVEMENTS OF CURVATURE 



which occur every time the circumnutating apex presses itself against the 

 support and drags upon the portion of the stem below, tightening the 

 coils, and drawing the stem over and around the surface of the support. 

 A pull of this kind is actually exercised every time the apex is pressed 

 against the support, and the periodic changes of tension set up in the 

 coiled but still growing parts by the movements of the free apex act in 

 the same way. Although these factors must aid in twining, it is not 

 certain whether they are the only ones acting. Baranetzsky in fact found 

 that Dioscorea Batatas was able to twine in their absence. It is, however, 

 readily possible that regulatory actions come into play of such character 

 as always to cause the side of the stem which is not in contact to grow 

 more actively, so producing the coiling around the support 1 . A regulation 

 of this kind is in fact essential to maintain a homodromous curvature of 

 the free apex, that is a curvature in the direction of twining 2 , for as the 

 result of circumnutation the permanently concave anterior side is continually 

 changing. Even according to Schwendener's theory twining is a physio- 

 logical manifestation produced by the plant exercising 

 a definite power of movement. 



We have no grounds for assuming that the factors inducing 

 circumnutation are quite unaffected by the commencement and 

 continuance of twining, and indeed the contrary is indicated by 

 the fact that circumnutation is not performed during all stages 

 of development. Similarly, it is uncertain whether the changes of 

 tone which produce circumnutation continue on a klinostat, on 

 which Cuscuta loses its contact irritability. The normal twining 

 of this plant indeed involves a periodic inhibition of the contact 

 irritability. 



Sachs 8 observed that growth was often distinctly retarded 

 in the free ends hanging beyond the support, or in shoots 

 which were unable to climb. According to Raciborski 4 , many 

 tropical climbers which fail to reach a support either throw 

 off their leaves or show a retarded development, followed 

 ultimately by the death of the growing point. Baranetzsky found 

 that the duration of growth was decreased in the twining portion 

 of the stem and the circumnutation diminished 6 . 



Unfavourable conditions, or the abscission or fixation 

 FIG. 8. Free coiling of of the free apex, tend to cause a production of free coils 



HumulusLufiulus. 



1 Baranetzsky, 1. c., p. 38 ; Noll, Sitzungsb. d. Niederrhein. Ges. f. Natur- u. Heilkunde, 4. Febr. 

 1895; Strasburger's Lehrb. d. Botanik, 1898, 3. Aufl., p. 225. 



3 De Vries, 1. c., pp. 336, 341 ; Darwin, 1. c., p. 19 ; Baranetzsky, 1. c., pp. 16, 65 ; Schwendener, 

 1. c. (1882), p. 436 ; Kolkwitz, Ber. d. bot. Ges., 1895, p. 513. 



3 Sachs, Lectures on Physiology. 



4 Raciborski, Flora, 1900, p. 2. These shoots have no contact irritability. 

 8 Baranetzsky, 1. c., p. 61 seq. 



