110 LIVERPOOL BIOLOGICAL SOCIETY. 
A very striking illustration of this plasticity of function 
is afforded by some recent remarkable observations by 
Dr. G. Klebs on the water-net, Hydrodictyon utriculatum, 
which, as far as [ am aware, have not yet been published 
in any English journal. Hydrodictyon exhibits both kinds 
of multiplication,—the conjugation of zoogametes, and 
non-sexual propagation by zoospores; and, in ordinary 
course, presents an illustration of the phenomenon known 
as alternation of generations; z.e., after a series of non- 
sexual zoospore-generations, the cycle is completed by the 
production of a sexual generation, viz., of zoogametes 
which conjugate to produce a zygote. In any individual 
Hydrodictyon-net, all the cells are apparently equivalent, 
i.e., are adapted to produce either zoospores or zoogametes. 
Klebs found that the tendency to produce one or the other 
of these structures is largely a question of nutrition; and 
that, in a single net consisting of equivalent sister-cells, 
some of the cells can be excited, by external conditions, 
to develop zoospores, others to develop gametes; the pro- 
duction of the former being, in all cases, absolutely 
dependent on light. His general conclusion is that there 
is not in Hydrodictyon any true and necessary alternation 
of sexual and non-sexual generations, such as is displayed 
in the Muscinee and Vascular Cryptogams; but that every 
cell of the net has the capacity for producing either kind of 
organ; and that it depends on external conditions which 
of the two forms of reproductive organ is brought into 
existence; favourable conditions tending, as a rule, to the 
production of non-sexual, unfavourable conditions to the 
production of sexual organs. A similar law, that a super- 
abundant supply of nutrition is favourable to the develop- 
ment of the vegetative or alimentary, at the expense of the 
reproductive organs, 1s not unknown in the higher branches 
of the vegetable, nor, I believe, in the animal kingdom. 
