THE PROGRESS AND MODE OF MOVEMENT 237 



of Barley to 5 kilograms, and to no less than 1 30 kilograms in the case of a Maize- 

 stem *. Meischke also measured the maximal pressure exercised by curving organs 

 against fixed resistances, and found that usually the basal nodes are able to take part 

 in the geotropic erection of the shoot. In the case of Avena, however, they only 

 begin to curve when the upward bending of the more apical portion has lessened the 

 statical moment exercised upon them. The internal resistance increases as the 

 curvature progresses, so that less external energy of movement is available, and 

 in the haulms of most Grasses complete erection requires the co-operation of 

 several nodes. 



Thin tendrils naturally are incapable of exercising any pronounced pressure 

 when curving, and in the case of the stiffer tendrils of Bauhinia, Strychnos, Vanilla^ 

 and other plants, a considerable internal resistance must be overcome before any 

 considerable external pressure can be exercised. Coiled hooks and tendrils which 

 undergo secondary thickening exercise sufficient pressure to strangulate the branches 

 they have clasped 2 , and become extremely rigid, whereas relatively thin fruit- and 

 flower-stalks (Apple, Snowdrop, Fuchsia) are mechanically bent by the weight of the 

 organ they support 8 . The peduncles of the Poppy are able to support the rather 

 heavy bud, and hence can perform active geotropic curvatures both negative and 

 positive in character. Most peduncles are, in fact, rigid enough to support the 

 flowers and flower-buds in any position, whereas the fruits, especially when succulent 

 and heavy, naturally tend to assume a more pendent character. 



The downward curvature of the root is always an active one, although Knight 4 , 

 Hofmeister 5 , and more lately Saposchnikow 6 and Letellier 7 , have considered it to be 

 a passive plastic bending produced by the root's own weight. This obsolete idea is, 

 however, sufficiently disproved by the fact that the root may curve against resistances 

 equivalent to more than its own weight 8 , and that it may curve downwards into 

 mercury against an upthrust of about ten times the weight of the part submerged 9 . 



A free root cannot exercise any great pressure owing to the readiness with which 

 it becomes laterally displaced, and because of the plastic properties of the growing 



1 Pfeffer, Druck- u. Arbeitsleistungen, 1893, p. 395; Meischke, Jahrb. f. wiss. Bot., 1899, 

 Bd. xxxin, p. 337. On the capacity for work in geotropically excited pulvini cf. Pfeffer, Periodische 

 Bewegungen, 1875, p. 145. 



2 Ewart, on Contact Irritability, Ann. du Jard. bot. de Buitenzorg, Vol. xv, 1898, p. 187. 



3 Cf. Vochting, Die Bewegungen d. Bliithen u. Friichte, 1882, p. 192 ; Wiesner, Sitzungsb. d. 

 Wien. Akad., 1902, Bd. cxi, Abth. i, p. 744. 



4 Knight, Phil. Trans., 1806, I, p. 104. Bazin appears, according to Duhamel, Naturgesch. d. 

 Baume, 1765, Bd. II, p. 109, to have attempted a similar explanation. 



5 Hofmeister, Jahrb. f. wiss. Bot., 1863, Bd. ill, p. 102 ; Bot. Ztg., 1868, p. 273, and 1869, p. 57. 

 Wigand suggested (Botan. Unters., 1854, P- 3) that the downward curvature of the part was due to 

 the distensive enlargement of the cells on the under side, but Hofmeister has shown that this is not 

 the case (Jahrb. f. wiss. Bot., Bd. Ill, p. 80). 



6 Saposchnikow, Bot. Jahrb., 1887, Bd. I, p. 225. 



7 Letellier, Essai de statique ve"ge"tale, 1893. 



8 Johnson, Linnaea, 1830, Literaturberichte, p. 148; Frank, Beitrage z. Pflanzenphysiologie, 

 1868, pp. 21, 35 ; N. J. C. Mtiller, Bot. Ztg., 1871, p. 719 ; Sachs, Arb. d. bot. Inst. in Wtirzburg, 

 1873, Bd. I, p. 450; Pfeffer, Druck- u. Arbeitsleistungen, 1893, p. 271; Wachtel, Bot. Centralbl., 

 1895, Bd. LXIII, p. 309 ; Meischke, Jahrb. f. wiss. Bot., 1899, Bd. xxxin, p. 366. 



9 Sachs, 1. c., pp. 431, 451. 



