236 
sensitive to contact and may be deflected, but not caused to 
coil, by it. 
The aerial roots develop on the under ventral moister side 
of the stem, shaded from light and turned away from the dor- 
sal (upper) surfaces of the leaves. If the stem is erect, the 
roots always develop from the shaded side. Hence the dorsi- 
ventrality is not induced by gravity. The stem is ageotropic, 
and in weak diffuse light, heliotropic, but with strong illumina- 
tion, dia-heliotropic. If the erect or horizontal stem is twisted, 
so that the shaded ventral side becomes the dorsal] exposed 
side, the roots for a time to the number of 2 or 3 may con- 
tinnue to develop from the old ventral side now exposed to 
light, but after growth in length has taken place, from the new 
portions of the stem the roots develop again from the under 
surface, and the leaves twist so ‘as to direct their dorsal surfa- 
ces upwards, the former dorsal surface of the stem being now 
physiologically ventral. Commonly, before any change takes 
place, at an intermediate node two roots develop, one from the 
dorsal and one from the ventral surface, indicating the point 
Where the reversal is taking place. The inherent tendency to 
dorsiventrality is therefore rendered definite b y the action of 
light and cannot be reversed when once induced , it being only 
in the new tissues and organs developed under the changed 
conditions, that a reversal of the previous dorsal and ventral 
surfaces may take place. | 
The young growing root tendril commences under favourable 
conditions to bend around a support in less than one day, and 
mm 2 days a very distinct curvature, which in 3 to 5 days may 
amount to a complete coil, is produced. On placing in alcohol 
and then in water, no further bending is produced in the first 
case or perceptible straightening in the latter. This is however 
also the case with hook-tendrils, or with tendrils in which the 
contact curvature is only slowly produced. For complete coiling 
and attachment, the roots of Vanilla may require as much as 2 
to 3 weeks. As the root grows older the epidermis dries and 
shrivels, and the layer beneath develops on the sides and outer 
