430 New combe, — The Sensory Zone of Roots . 
tropism 1 , and probably heliotropism 2 and hydrotropism 3 . 
Thermotropism 4 , aerotropism 5 , galvanotropism 6 and rheo- 
tropism 7 , are responses known to be due to the sensitiveness 
of the elongating zone, but in these cases the root- apex is 
not known to be insensitive. Thigmotropism 8 , as such, has 
as yet no place among phenomena of irritability in any but 
aerial roots, for Wiesner 9 and Spalding 10 have shown the 
thigmotropism of Darwin to be traumatropism, and I shall 
shortly offer evidence to show the same thing regarding 
Sachs’ thigmotropism. 
The foregoing statement is enough to indicate that in roots 
no sensory area has been found posterior to the elongating 
zone. The following pages will show that several plants 
possess sensitive areas removed by a considerable distance 
from the elongating zone. Evidence will also be offered to 
show that the rheotropic sensitiveness of the root-apex of 
Bras sic a alba is to be assumed as well as that of Raphanus 
sativus and Zea Mays. 
II. The Rheotropic Sensitiveness of the Apex 
of the Root. 
In the paper 11 already referred to, experiments were de- 
tailed in which the rheotropic sensitiveness of the apical 
millimetre of the roots of Helianthus annuus , L., and Brassica 
alba, Boiss., was made probable by the application of fine 
1 Wiesner, Sitzb. d. k. Akad. d. Wissensch. Wien, Bd. lxxxix, I. Abth., 1884. 
Also Spalding, Ann. of Bot., viii, 1894, p. 423. 
2 Kohl, Die Mechanik der Reizkriimmungen, p. 26. 
3 Molisch, Sitzb. d. k. Akad. d. Wissensch. (math.-nat. Cl.), Wien, Bd. lxxxviii, 
I. Abth., 1883, p. 897. Also Pfeffer, cited by Czapek in Jahrb. f. wiss. Bot., xxxv, 
p. 316. 
4 Wortmann, Bot. Zeit., xliii, p. 232. 
5 Molisch, Sitzb. d. k. Akad. d. Wissensch. (math.-nat. Cl.), Wien, Bd. xc, I. 
Abth., 1884, p. 175. 
6 Brunchorst, Ber. d. D. Bot. Gesellsch., ii, 1884, p. 204. 
7 Juel, Jahrb. f. wiss. Bot., xxxiv, 1900, p. 518. 
8 Darwin, Power of Movement in Plants, p. 129. Also Sachs, Arbeit, d. Bot. 
Inst. Wiirzb., i, p. 437. 
9 Wiesner, loc. cit. 
11 Newcombe, Bot. Gaz., xxxiii, 1902, p. 177. 
10 Spalding, loc. cit. 
