Wager . — The Perception of Light in Plants. 
481 
still found to be heliotropically sensitive. He points out, for example, that 
in Acer A egnndo and Cornus sanguinea there are sometimes whole shoots in 
which the leaves are almost bleached and destitute of chlorophyll. These 
show, notwithstanding the absence of chlorophyll, in their young stage, 
a dia-heliotropic response, which is, however, in most cases lost as the leaves 
become older. The weak point in this argument seems to me to be that, 
although the leaves are destitute of chlorophyll, they may contain, at any 
rate in the earlier stages, a certain amount of yellow colouring matter 
by which the more refractive rays of the visible spectrum would be 
absorbed, and thus satisfy the requirements for a heliotropic response. 
Light is able to bring about an orientation of the chloroplasts in many 
foliage leaves and in some Algae. Under the influence of too strong 
or too weak a light, they are brought into 
more suitable positions, either by turning 
completely round, as do the chlorophyll 
bands of Mesocarpus , or by moving into 
different positions in the cell, as in Elodea , 
Oxalis ace to sella, Schistostega osmundacea, 
&c. Just as in the orientation of the leaf 
itself, the stimulus which brings about the 
orientation of the chloroplasts is produced 
by the more refrangible rays of the spectrum 
absorbed by the chlorophyll. 
Light sensitive motile organisms, such 
as Euglena and the swarm spores of Algae, 
behave in red light just as they do in the 
dark, but exhibit pronounced heliotaxis in 
light blue. This is the more significant as 
the red spot, which is supposed to function 
in this heliotactic stimulation, absorbs just 
those rays which are active. 1 In these cases, 
therefore, it is fair to assume that the move- 
ment is brought about by absorbed rays and 
not by any kind of mechanical action on the cytoplasm. The association 
of the spherical or oval swelling on the basal portion of the flagellum in 
Euglena^ with the red pigment-spot (Text-Fig. 2), is analogous to the asso- 
ciation of the rods and cones of the animal eye with their pigment layer. 
The light absorbed by the pigment-spot probably sets up chemical changes 
which affect the swollen part of the flagellum, and this in turn acts upon the 
Text-Fig. 2. Side view of the 
anterior end of a cell of Euglena viridis, 
showing the flagellum and its enlarge- 
ment in front of the eye-spot. (From 
Journ. Linn. Soc. Zool., xxvii, 1899.) 
1 Wager: On the Eye-spot and Flagellum in Euglena viridis. Journ. Linn. Soc. Zool., xxvii, 
463, 1900. Mast: Light Reactions in Lower Organisms, II. Volvox. Journ. Comp. Neurology and 
Psychology, xvii. 112, 1907. 
