484 Wager. — The Perception of Light in Plants. 
grains is sufficient to account for a direct movement towards a right position, 
without the intervention of trial and error movements, is perhaps not so 
certain as in the cases of clear centric illumination of the basal walls of the 
epidermal cells described by Haberlandt ; but it would take place quite as 
surely and as readily as in those cases of unequal non -centric differential 
illumination which, as I have shown (p. 468), occur in a large number of 
leaves with epidermal lens cells of long focus ; and it certainly affords 
a much more satisfactory explanation for those cases in which the epidermal 
cells are flat or possess only curved basal walls. It would also enable us to 
account for those cases in which a heliotropic response takes place when the 
lens fuction is eliminated. Even if we have to assume a certain amount of 
trial and error movement before the right direction is perceived, and I am 
A B 
Text-Fig. 3. Diagram showing how the chlorophyll grains in the palisade cells of a foliage 
leaf with flat outer walls would be partially illuminated in oblique light. A and B show the 
difference in the illumination at different angles of incidence. 
not sure that this is not also necessary in Haberlandt’s hypothesis, we have 
a basis for this in the autonomous movements of circumnutation which have 
been so fully described by Charles and Francis Darwin 1 . They point out 
(p. 3) that : — c The movements of various organs to the light, which are so 
general throughout the vegetable kingdom, and occasionally from the light, 
or transversely with respect to it, are all modified forms of circumnutation.’ 
And again (p. 419) — ‘ Heliotropism seems always to consist of modified 
circumnutation. Any kind of movement in relation to light will obviously 
be much facilitated by each part circumnutating or bending successively in 
all directions, so that an already existing movement has only to be increased 
in some one direction, or to be lessened or stopped in the other directions, 
in order that it should become heliotropic, apheliotropic, &c., as the case 
may be.’ 
1 The Power of Movement in Plants. London, 1880. 
