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Francis Darwin. 
regarded from the point of view of adaptation. A leaf happens 
to be both plagiotropic and dorsiventral, but these qualities are 
distinct adaptations to illumination. A lateral root is plagiotropic 
in adaptation to space relations, but has no need of dorsiventrality 
in an environment which is the same above and below. 
With regard to minor details it is no great matter whether we 
distinguish the different kinds of geotropism and heliotropism by 
the symbols + and —, or by the affixes pros and apo , although 
personally 1 prefer the latter which were suggested in the “ Power 
of Movement.” In the same way it is of no importance whether 
we speak of phototropisiu or heliotropism. The newer form is 
somewhat pedantic, but it has the merit of brevity and seems to be 
supplanting De Candolle’s original term. For the same reason and 
for brevity’s sake I prefer dia-geotropism to transverse geotropism, 
and I give a similar preference to dia-heliotropism. 
With regard to the tactic movements of swimming organisms 
it has been already pointed out that they may, roughly speaking, be 
classed with the tropic curvatures because of tbeir directive 
character. The question whether in the case of phototaxy the 
direction of the light is decisive was made clear by Elfving in an 
interesting paper. 1 Oltmanns 2 believed himself to have proved on 
the contrary that phototactic movements are dependent merely on 
the intensity of the light. 
The experiments were conducted in a glass vessel rendered 
opaque on three 3 sides, the fourth side being made of a glass 
prism filled with a dilute mixture of gelatine and Indian ink, as 
shown below. 
Birdseye View of Oltmanns’ Trough. 
1 Finska Vet. Soc. Fdrhand. Feb., 1901, Vol. XLIII. 
2 Oltmanns in Flora, 1892. 
3 Oltmanns’ apparatus had prisms on both of the larger sides but 
the form given in the Figure will serve equally well for purposes 
of explanation. 
