Nomenclature and Method. 
229 
If we consider the light striking the box at right angles to its 
longer axis, it is clear that at D the light will be greatly diminished 
by passing through a thick layer of blackened gelatine, while at L 
the illumination will be much brighter, and at M there will be an 
intermediate condition. Volvox is very sensitive to differences in 
illumination and seeks out an optimum intensity ; thus the organisms 
collect at M, moving from D where the light is not strong enough, 
and from L where it is too strong. In doing so they appear to move 
across the direction of incident light. But according to Elfving 
this is an error depending on the assumption that all the light enters 
the chamber at right angles to the transparent wall. Elfving points 
out that this is not so; an organism at M will be subject to strong 
oblique light from the right and weak light from the left, the 
resultant of these will be oblique and from the right. The organism 
will, in moving in the line of the resultant, travel obliquely across the 
long axis of the chamber and in doing so will pass through regions 
of varying intensity of light, in one of which it will come to a halt. 
It is only necessary to imagine that light is entirely excluded 
at D to see that a Volvox at the darkened end in moving towards 
or from the light, would travel more or less in the line of the long 
axis of the chamber. 
Chemotaxy is undoubtedly a case of directive action, that is to 
say it resembles a tropic rather than a nastic curvature. It is 
moreover usually comparable to orthotropic action ; the antherozoids 
of ferns (for instance) may be seen to turn in their course and rush 
straight at the capillary tube from which malic acid diffuses. But 
this is not the only manner in which chemotaxy takes place. 
Rothert 1 has described the different behaviour of certain Bacilli 
which were large enough to be easily followed under the microscope. 
They do not make a directive approach to the source of attraction ; 
if, however, they chance to come near it they do not leave it again 
but remain swimming backwards and forwards close to it. They 
are not stimulated to move in the line of increasing concentration, 
but a dimination of concentration affects them in a curious way : 
they stop suddenly and swim backwards until they again reach a 
region of falling concentration, when the same thing occurs. Thus 
they are forced to remain near the diffusing object, but, as Rothert 
says, the attraction is only apparent. He calls this apobatic 
chemotaxy, the ordinary type being strophic. 
Engelmann had previously observed a similar effect with 
1 Flora, 1901. 
