THE PROGRESS AND MODE OF MOVEMENT 235 



upon the degree of irritability, and the latter may not be fully developed 

 in the most actively growing zones, or may be entirely absent from them 

 even when the powers of perception and reaction are not localized. The 

 maximum irritability appears commonly to be attained by nodes after the 

 grand period of growth has been passed, and this has been definitely proved 

 to be the case in the nodes of Tradescantia by Barth and Kohl ] , while the 

 nodes of Dianthus banndtictis, and of a few grasses, only acquire their 

 special geotropic irritability after their normal growth has ceased. It is 

 owing to changes in the distribution of the altered irritability that the 

 positively heliotropic curvature of the stem of Tropaeolum majus takes place 

 mainly in the zone of most active growth, whereas the negatively heliotropic 

 curvature is performed by the older but still growing regions. The fact 

 that in other cases the heliotropic curvature begins in the most actively 

 growing zone affords no evidence of the existence of two special kinds 

 of negative heliotropism as suggested by H. Muller 2 . Since tropic 

 irritability is always lost beyond a certain stage of development, it is 

 possible that in certain cases it may disappear before growth in length 

 has ceased, although in all the plants hitherto examined the whole growing- 

 zone remained irritable. 



In the case of the haulms of grasses, two or more nodes co-operate in 

 producing the geotropic upward curvature of a horizontally-placed stem, 

 since the internodes are inactive, and a single node is unable to curve 

 sufficiently to make the stem erect. Other plants which possess motile 

 nodes behave similarly, the geotropic response being performed mainly 

 or entirely by the nodes. The special geotropic irritability of the nodes 

 of Mercnrialis was observed by Bonnet 3 a century and a half ago, but the 

 general nature of the phenomenon was only established by the researches 

 of de Vries 4 and later authors, while Wiesner 5 has investigated the helio- 

 tropic irritability of the nodes of certain plants. 



The rapidity of reaction. This is most pronounced in the case of 

 sensitive tendrils, for they may perform a considerable curvature in a few 

 minutes when thigmotropically excited. It takes one or more hours for 

 a thin actively growing stem to become erect when geotropically excited, 

 while thicker or less irritable stems may require one or more days to attain 



1 Earth, Die geotropischen Wachsthumskrummungen d. Knoten, 1894, p. 19; Kohl, Mechanik 

 d. Reizkrlimmnngen, 1894, p. 21. 



2 H. Muller, Flora, 1876, pp. 70, 93. 



3 Bonnet, Nutzen d. Blatter, 1762, p. 68. 



* De Vries, Landw. Jahrb., 1880, Bd. IX, p. 473 ; Rutzow, Bot. Centralbl., 1882, Bd. IX, p. 81 ; 

 Briquet, Monographic du Genre Galeopsis, 1893, p. 60; Barth, Die geotropischen Wachsthums- 

 kriimmungen der Knoten, 1894; Kohl, Bot. Ztg., 1900, p. I (Tradescantia) ; Westermaier, Ueber 

 gelenkartige Einrichtungen an Stammorganen, 1901 ; Miehe, Jahrb. f. wiss. Bot., 1902, Bd. XXXVII, 

 P- 527 (Tradescantia). 



5 Wiesner, Die heliotropischen Erscheinungen, 1880, Bd. n, p. 33. 



