HAPTOTROPISM 



499 



may become convex, and it has yet to be determined whether the inner side can 

 be rendered convex under appropriate experimental conditions. It is only as 

 a result of indirect stimulation that a tropistic curvature is induced, the curvature 

 in the other case is nastic. By nastic curvatures we mean (compare p. 428) 

 those whose direction is determined by the plant itself. A nastic curvature may, 

 however, be induced just as well by a stimulus directed in a definite direction as 

 by diffuse stimuli. Which process takes place in Drosera is not quite established, 

 but it is very probable that the stimulus first affecting the apex passes with 

 equal intensity to all parts of the stalk, but one side (the upper or the under) in 

 all likelihood is more sensitive than the other, a view which is supported by a 

 comparison with the behaviour of tendrils. These organs behave purely tropisti- 

 ca//y after contact stimulation, after chemical and thermal stimuli, however, they 

 behave nastically. The curvature which is induced by heating or cooling is, 

 according to Correns (1896 a), always in the same direction ; it is the under side 

 always that exhibits concave curvature, beginning from the apex in the case of 

 the tendril, and the result is the same whether the temperature of the tendril 

 is, by conduction, altered to a similar extent on all sides, or whether it be heated 

 on any one side by radiation. We must therefore conclude that heating of any 

 part of the tendril results in a uniform rise of temperature and that the conse- 

 quent movement is always the same, viz. that determined by the physiological 

 dorsiventrahty of the tendril. At the same time it is very remarkable that 

 tendrils which react to contact equally well all round always curve so that only 

 their under sides become concave when subjected to heat stimulus. [The 

 mechanics of tendril curvature following on heating does not differ from that 

 resulting from contact stimulus ; the same is true of curvature induced by 

 wounds, save that in that case the transmission of the stimulus is very rapid 

 and far-reaching (Fitting, 1903 b).] 



Drosera (and the tendril also) presents us with very interesting transitions 

 between tropistic and nastic reactions. Following on mechanical and chemical 

 stimulation, very delicate nastic movements, which are closely related to those 

 of Drosera, occur also in other carnivorous plants, as for instance in Dionaea 

 muscipula, Aldrovanda vesiculosa and, less markedly, in Pinguicula. Detailed 

 research on these forms, comparable to those which have been made on Drosera, 

 have not as yet been carried out, though much required, so that we had best 

 content ourselves with this brief reference ; still less have we space to discuss 

 haptotropic curvatures in lower forms (Mucorinae, Errera, 1881 ; Trzebin- 

 SKY, 1902 ; Algae, Nordhausen, 1900). 



Bibliography to Lecture XXXVIII. 



Baranetzky. 1 90 1. Flora, 89, 138. 



Batalin. 1877. Flora, 60, 36. 



Correns. 1896 a. Bot. Ztg. 54, i. 



Correns. 1896 b. Ibid. 54, 21. 



Darwin, i 876 a. Die Bewegungen u. Lebensweise kletternder Pflanzen (Carus). 



Stuttgart. 

 Darwin. 1876 b. Insektenfressende Pflanzen (Carus). Stuttgart. 

 Errera. 1884. Bot. Ztg. 42, 497. 



Ewart. 1898. Annales Jard. bot. de Buitenzorg, 15, 187. 

 Fitting. 1902. Ber. d. bot. Gesell. 20, 373. 

 Fitting. 1903 a. Jahrb. f. wiss. Bot. 38, 545. 

 [Fitting. 1903 b. Ibid. 39, 424.] 

 Haberlandt. 1 90 1. Sinnesorgane i. Pflanzenreich z. Perzeption mechanischer 



Reize. Leipzig. 

 MacDougal. 1896. Ber. d. bot. Gesell. 14, 151. 

 MxJLLER, O. 1887. Cohn's Beitr. z. Biol. 4, 97. 

 [N£mec. 1904. Beihf. bot. Centrbl. 17, 52.] 

 [Newcombe. 1902. Beihf. bot. Centrbl. 12, 243.] 



K k 2 



