OF MICROSCOPICAL PLANTS. 259 
no movement towards the edges takes place. In a tall vessel 
of water, which is lighted by the daylight coming from above, 
as usual, the organisms move upwards towards the surface of 
the water, and in the same way in ponds in the open air; 
on the other hand, by lighting only one side in a room, they 
go to the upper edge, which is the nearest to the window, 
and which is turned towards the source of the light. 
If, on the contrary, we allow the light to fall “underneath; 
or from a point in the side of a cylindrical vessel of water, 
in the organisms in the first case will move downwards and 
the latter - sideways towards the source of light. 
As soon as the light from above is moved away the 
organisms may be moved to any point by reflected light ; for 
example, in a flat drop on a glass slip, when the rays fall 
parallel from underneath, they move equally downwards to- 
wards the bottom of the drop. By placing the reflector in 
an oblique position they are made to move towards the cor- 
responding edge of the drop. If the reflector of the micro- 
scope presents a definite image of the crossbars of the window 
on the object-plate the green organisms arrange themselves 
accordingly, leaving the darker crossbars empty, and covering 
those liquid parts which correspond to the glass, thus giving 
a negative picture of the window on the object-plate. 
From these and a great number of analogous experiments 
the following conclusions are drawn: 
(1) The direction of the movements of green microscopical 
organisms is determined by the direction of the rays of light 
which fall upon them. The organisms moye towards the 
source of light, exactly contrary, as if it were to the direction 
of the rays of ight themselves. They are, as well as we can 
express it, attracted rectilineally by the source of light.. Ap- 
parent exceptions to this rule are brought about simply by 
the form of the drop or mass of water in which the organisms 
are. 
(2) These green organisms exhibit a polar relation to light; 
they place themselves always so that one half of the body, 
which is generally characterised by the want of chlorophyll, 
as well as by the attachment of cilia (which is called the 
head), is turned towards the source of light, and the opposite 
green half of the body (tail) is turned away from the source 
of light. When the light is shut off no particular position is 
assumed. 
(3) All movements of green organisms are accompanied 
by a rotation of their bodies round the longitudinal axis pass- 
ing through the head and tail. Whilst in the dark the organ- 
isms can turn from left to right, as well as from right to ‘eft, 
