IN VEGETABLE BIOLOGY. 239 
case, but to a limited extent only. The reason for this seems 
to be as follows. In the darkling plants the early setting-in 
of rotation is favoured by the lowering of the protoplasmie 
tone by darkness and by the poison; opposed to this is the 
stored-up and only slowly-soaking-out action of light tending 
to maintain epistrophe. The lowering effect of the poison is, 
in the illuminated specimens, operant to the same degree as 
before, and it is reinforced at night by the action of darkness; 
but during the daytime this latter action is, if the light be 
not too strong, exchanged for the epistrophizing tendency of 
medium diffused light. It is clear that the light used must not 
be too intense; otherwise, instead of favouring epistrophe, it will 
take part in driving the protoplasm and chlorophyll on to the 
lateral walls, which it will be enabled more easily to do from the 
lowering in its tone experienced by the protoplasm as a conse- 
quence of poisoning. It was perhaps from experimenting with 
light of too high quality that in some cases no appreciable differ- 
. ence could be detected in this matter between darkling and ex- 
posed specimens ; but I hope to obtain further information upon 
this point. It usually happens that the death-stasis, into which 
rotation from poisoning passes, ensues more rapidly in darkness 
than in light; this is only what would be expected, seeing that 
rotation from poisoning makes an earlier appearance in the dark, 
in addition to which the absence of light is a factor favouring the 
cessation of vitality. 
The analogy of the epistrophic may be applied to the construc- 
tion of a “ eyclolytie ” interval, the left-hand end of which will be 
very near to the positive critical point, without actually reaching 
it however, because, in poor light of just the requisite intensity, 
it is possible to apostrophize the chlorophyll of Elodea without 
any rotation supervening even after several hours. Towards the 
right the cyclolytie interval will extend as far as that degree of 
concentrated sunlight at which the movements of protoplasm 
are arrested. Moreover, the extent of the interval will vary with 
the protoplasmic tone, any lowering of which will be accom- 
panied by the former's leftward development: it will also depend 
upon the position of the cell ; thus the interval of the upper cells 
of Elodea-leaves will extend further to the left than will that 
of those of the lower layer. 
