176 Wager: Movements of Aquatic Micro-Organisms. 
become more brisk. These various responses appear therefore 
to be due. in part, to the variation in the intensity of the light. 
Many observers have tried to show that light intensity 
plays no part in the stimulus, but that it is simply the direction 
of the rays of light that is effective. They assume that the 
stimulus acts directly on the motile organs, and that the axis 
of the organism is oriented into such a position that it is acted 
upon symmetrically on all sides by the external force and is 
thus kept in a definite position. It is probable however that 
neither the heliotropic bending of fixed plants nor the helio- 
taxis of motile organisms can be explained in this simple 
mechanical fashion ; the stimulus seems to be interpreted by 
the living protoplasm in some way not at present understood, 
and the resulting movement or modification in the direction 
of movement depends upon the nature of this interpretation. 
It is obvious that the movements of the organism, as well as 
any change in the direction of its locomotion, are dependent 
directly upon the flagellum. Without it the cell is only 
capable of a very slow contractile movement of its body. The 
movements of the flagellum undoubtedly originate in the 
release of energy already stored up in the cell and are not 
directly due to an external stimulus acting during the time 
the movements are taking place, but there can be no doubt 
I think that any change in the direction of locomotion must 
be due to chemical and physical changes set up in the proto- 
plasm by external stimuli such as light and heat. This does 
not mean that the responses to these stimuli must therefore 
be mechanical, or that they are necessarily to be regarded ‘ as 
an expression of conscious sensations, discrimination or 
will.’ Our knowledge of the phenomena is not sufficient or 
precise enough to warrant a strong support of either view at 
present. Possibly the truth lies somewhere between these 
two extreme views. There is very little evidence that the 
response is a conscious one, but on the other hand there is just 
as little evidence that the response is of the nature of a tropism 
or a purely mechanical reaction to an external force. We 
shall see later that the response of Euglena to gravity is purely 
mechanical, but the response to light is much more complex, 
and more like an ordinary reflex action. We may look upon it 
as the result of the unconscious adjustment of the organism to 
the environment, and dependant upon the protoplasmic 
constitution, or as Jennings terms it, the ‘ physiological state’ 
of the cell. 
That the nature of the response to light depends upon the 
physiological state of the cell may be inferred from the varia- 
tions which are to be observed in the behaviour of organisms 
under varying temperature conditions. Thus under certain 
conditions which are not clearly understood, Euglena and 
Naturalist, 
