260 COHN, ON MICROSCOPICAL PLANTS. 
and often change these directions; but a definite direction of 
rotation is.given to them by the light. In Euglena and some 
other organisms this is a contrary course to that of the hands 
of a watch, but the same as the rotation of the earth. 
(4) Experiments with coloured glass show that only the 
more highly refractive actinic rays induce this direction of 
movement ; the less refractive rays, which have no chemical 
activity, are simply negative in action, as in the absence of 
light. The organisms are attracted most strongly by the blue 
rays, whilst the red are the same as total darkness. Thus, 
for example, if half the field be lighted by blue, and the other 
half by red light, the organisms will all go to the blue, al- 
though it be turned away from the window-edge. 
(5) By far the majority of green organisms follow the laws 
here laid down. There are, however, great numbers of ex- 
ceptional forms which turn away from the source of light by 
a backward motion. In these organisms the rotation along 
the longitudinal axes is reversed, and there is a point, sooner 
or later, where they suddenly stop in their backward moye- 
ment and stand still for some time, and then, by changing 
the direction of rotation, go over again towards the source of 
light. 
(6) If we consider these facts concerning the movements of 
organisms which possess a green and a colourless half in con 
nection with the property of chlorophyll to effect, through 
the agency of actinic rays, certain chemical actions—in par- 
ticular the decomposition of carbonic acid and the separation 
of oxygen—it appears probable that all these phenomena of 
movement, as far as concerns their direction being caused by 
light, depend upon the chemical activity of these bodies. We 
can, in fact, imitate, by pure chemical processes, with the 
help of what may be called an artificial Euglena (namely, 
a fusiform fragment of chalk, half of which is covered with 
a resinous cement, and which is placed in diluted sul- 
phuric acid), many of the phenomena recorded above. 
The splinter of chalk develops oxygen on its uncovered half, 
and is thereby projected by the backward impulse in the 
direction of the covered end, and is caused to rotate. 
