THE MECHANISM OF CURVATURE 243 



ceptive and responsive. Similarly the geotropic stimuli perceived by the 

 apex of the root appear to produce the most active response in the cortical 

 tissues 1 , and these, owing to their peripheral position, are more readily 

 capable of producing curvature than centrally placed ones. 



Sachs 2 found that the middle lamella cut out of a stem performed 

 a negatively geotropic curvature when placed horizontally with the cut 

 surface perpendicular, whereas when the cut surfaces face downwards or 

 upwards the results obtained vary and are often negative. The absence of 

 any curvature might possibly be due to the insufficient leverage exerted by 

 the thin slice of the cortex. Czapek found, however, that a horizontal slice 

 of the middle lamella of the hypocotyl of Helianthus annuus performed 

 a negatively geotropic curvature when the section was prepared after an 

 hour's previous geotropic excitation. This isolated observation does not 

 necessarily prove that the horizontally placed lamella is always able to 

 perform a geotropic response, but not to perceive geotropic stimuli. 



Both irritability and the power of response change during development, 

 and all tissues which have lost the power of growth can only experience 

 a passive curvature. In addition, the less active tissues may be compressed 

 or stretched in accordance with their position in regard to the more active 

 ones. The latter applies to the nodes of grasses in which the originally 

 active parenchyma tissue on the convex side is ultimately ruptured by the 

 continued growth of the collenchyma strands 3 . In this way the previously 

 compressed parenchyma is stretched, while the stretched collenchyma 

 becomes subject to compression. Evidently, therefore, the strains in the 

 tissues do not afford direct evidence as to the part each tissue plays in cur- 

 vature. In addition, every nutation curvature, and the tissue-strains to which 

 it gives rise, may co-operate in modifying the original growth-tendencies. 



Even when the pith has no active power of curvature, its compression 

 may aid in producing curvature when this is once initiated, but apart from 

 this mechanical action the detailed changes of the tissue-strains during 

 the progress of heliotropic and geotropic curvatures fail to reveal the 

 mechanism of curvature 4 . The anatomical differences between negatively 

 and positively tropic organs postulated by Dutrochet 5 were shown long ago 

 by Mohl 6 to be non-existent. Dutrochet also erroneously supposed that 



1 Macdougal, Annals of Botany, 1897, Vol. xxui, pp. 346, 364. 



2 Sachs, Flora, 1873, p. 330; Arb. d. hot. Inst. in Wiirzburg, 1873, Bd. I, p. 470; Czapek, 

 Jahrb. f. wiss. Bot., 1898, Bd. xxxil, p. 250; Noll, Jahrb. f. wiss. Bot., 1900, Bd. xxxiv, p. 467 ; 

 Haberlandt, Ber. d. hot. Ges., 1901, p. 270; Jahrb. f. wiss. Bot., 1903, Bd. XXXVIII, p. 470; 

 Nemec, Ber. d. bot. Ges., 1902, p. 353. 



8 Cf. Pfeffer, Druck- u. Arbeitsleistungen, 1893, p. 407. 



4 Sachs, 1. c. ; Frank, 1. c. ; Hofmeister, Pflanzenzelle, p. 293; Kraus, Bot. Ztg., 1867, p. 129; 

 Ratschinsky, Ann. sci. nat., 1858, 3* ser., T. IX, p. 172; Johnson, ibid., 1835, 2 a ser., T. IV, p. 327; 

 Pollock, Bot. Gazette, 1900, Vol. XXIX, pp. 25, 48. 



5 Dutrochet, Mdmoires, etc., Bruxelles, 1837, pp. 322, 327. 



6 Mohl, Vegetabilische Zelle, 1851, p. 141. 



R 2 



