233 
swaying movements caused by the wind etc. have given the 
hook every chance of releasing an object incapable of affording 
any marked support. Before, however, proceeding to any ge- 
neral comparisons, it will be as well to consider the pheno- 
mena of contact irritability as manifested by the aerial roots 
of certain tropical epiphytes. 
Mou. ') mentions that the “Wurzel-ranken” of Vanilla aromatica 
are often a foot long or more, may grow directly upwards, 
and may coil around a support. Darwin *) found that the roots 
were deflected by contact with a support perceptibly in 1 day 
and formed root hairs, but did not coil, and continued their 
downward course. Trevs*) noted the coiling and adhesion to 
flat surfaces, by the roots. Sacns‘) has shewn, that in roots in 
general, growth is retarded on the side in contact, and both 
Trevs and Sacns consider the coiling to be a manifestation of 
the same phenomenon. The unilateral retardation of growth, 
however, caused by contact, is in ordinary roots, provided no 
traumatropic curvature-is induced , insufficient to cause the for- 
mation of even a single complete coil. Prerrer *) considers that 
the coiling is due to the possession by the aerial roots of Vanilla 
of a special irritability similar to that possessed by tendrils. Went 
found with Pothos aurea that in darkness the roots grew straight 
downwards, and came to the conclusion, from their curvatures 
when placed in various positions, that they are also ahelio- 
tropic, and that the twining is due to a special contact irrita- 
bility, but without giving any definite experimental proof of 
the same. Wenr points out that two kinds of aerial roots are 
to he distinguished, namely long descending “Nahrwurzeln” 
and short twining or adherent “Haftwurzeln”, and shews song ea 
two kinds of roots probably possess very different physiological 
1) Mout, Ueher den Bau und das Winden der Ranken und Schlingpflanzen. 
Tubingen 1827, p. 48. ee 
2) D Pflanzen. p. : 
“4 Paice: nse bot. de Scisieauene Vol. 1H, 1883, p. 177 und 178, 
4) Sacus, Arb. d. Bot. Inst. Wurzburg. I, p. 438. 
5) Prerrer, Physiologie. Bd. II, p. 224, 1880. Went, 
Vol. XII, pl. 1, 1894, p. 28. , 
Ann. Jard. Buitenz. Vol. XV, J. 
Ann. Jar. bot. Buitenzorg. 
16 
