230 
Francis Darwin. 
Bacterium photo metricum} It is not attracted by light but cannot 
escape from a bright spot because a region of falling intensity 
makes it swim backwards with what has been called a fright or 
phobic movement (and this is perhaps a better term than apobatic). 
We may possibly see here a certain resemblance to nastism, since 
the direction of a phobic movement is morphologically determined, 
being a mere reversal of a previous action—a case of “ backing 
water” in fact. 
In conclusion I have to say a few words by way of introduction 
to my next lecture. We have seen that the principal tropic 
movements, namely those made in response to light and to gravity, 
may be looked at as parts of one phenomenon. But there is one 
striking difference between geogenic and photogenic action which 
especially concerns the methods of the investigator. I mean that 
he can at will remove the plant from the effects of light, but to 
free them from the action of gravity is not so easy. It is true that 
he can distribute the stimulus of gravity so that it ceases to be 
effective, and since I shall frequently refer to the klinostat by which 
this is effected it will be convenient to give a few introductory words 
on this instrument. The klinostat 2 is a clockwork arrangement by 
means of which a plant is kept in constant slow rotation. If the 
axis is vertical the effect is to diffuse, and thus destroy, the directive 
action of light. On the other hand if the axis is horizontal the 
directive action of gravity is in like manner removed. When Sachs 
was designing the klinostat he did not at first perceive that the 
horizontal instrument could be employed for both purposes—a fact 
which he afterwards made so familiar to us. If the axis of the 
klinostat is directed to the source of light the rotating plant 
will be subject to photogenic, but not to geogenic stimulation, 
while if the axis of rotation is at right angles to the line of illumi¬ 
nation the plant will shew neither geotropism nor heliotropism. 
The two theories of the action of the klinostat may be briefly 
stated as follows :— 
(i.) That the plant is never long enough in one position to be 
able to perceive the stimulus. 
(ii.) That the stimulus is perceived but that it affects equally 
all sides of the plant and is therefore ineffective. 
There can be no doubt that the second is the true theory, as 
will be shown in detail in the next lecture. A form of the instru- 
1 Pfliiger’s Archiv, 1882. 
2 Sachs in his Arbeiten ii. 
