48 5 
Wager . — The Perception of Light in Plants. 
So also, in discussing the connexion between circumnutation and 
positive heliotropism it is pointed out (p. 436) that : — ‘ A plant, when 
exposed to a lateral light, though this may be bright, commonly moves at 
first in a zigzag line, or even directly from the light ; and this no doubt 
is due to its circumnutating at the time in a direction either opposite to the 
source of the light, or more or less transversely to it. As soon, however, 
as the direction of the circumnutating movement nearly coincides with that 
of the entering light, the plant bends in a straight course towards the light, 
if this is bright/ 
In the case of dia-heliotropism, a similar explanation probably holds 
good ; ‘ cotyledons and leaves place themselves so that their upper surfaces 
may be exposed to the light, and this movement is regulated, though not 
directly caused, by the direction whence the light proceeds ’ (p. 442). 
In both orthotropic and diatropic organs, the final result aimed at — 
the equal illumination of the chlorophyll grains — is the same. Ortho- 
heliotropic organs are symmetrical ; dia-heliotropic organs are bi-sym- 
metrical. In the latter case the position of the chlorophyll-containing 
tissue, not the epidermis merely, must determine finally the light position 
of the leaf, just as, in the former case, the symmetrical distribution of the 
chlorophyll tissue must finally determine whether the organ shall place 
itself parallel to the rays of light, or laterally with respect to them. 
So far as the chlorophyll grains are concerned, the one-sided illumina- 
tion of ortho-heliotropic organs corresponds almost exactly with the oblique 
illumination of dia-heliotropic organs. The only difference is that, in the 
one case, the whole of the chlorophyll grains on one side of the organ are 
exposed to the light, while the other side remains in obscurity ; in the 
other case, the differential illumination takes place in each palisade cell in 
such a way that the chlorophyll grains on one side of the cell are more 
brightly illuminated than on the other. If this one-sided illumination 
is sufficient in the one case to bring about a regulation of the fundamental 
movements of circumnutation, it is fair to conclude that it is equally 
sufficient in the other. 
In dia-heliotropic leaves which possess papillae, or other contrivances in 
their epidermal cells, capable of causing a convergence of the light rays, we 
should still have a one-sided illumination of the chlorophyll grains, probably 
more efficient than in leaves with flat-walled epidermal cells. It is quite 
possible, therefore, that in such cases the lens cells, although not absolutely 
necessary, may be of advantage, especially to shade plants, in bringing 
about the light perception. 
