288 
Notes. 
moving protoplasm is brought to rest in a period varying from two to 
twelve hours, and that it seemed to make no difference whether light 
were excluded or not. I repeated the experiment on Nitella, but 
used pure hydrogen instead of the mixture of gases employed by 
Professor Pringsheim. Possibly the discrepancies existing between 
our observations may be partly due to this fact. Nitella and also 
Chara are somewhat difficult plants to work with ; and often, for no 
apparent reason, fail to give conclusive results unless one is very 
careful to select thoroughly healthy cells in which the rotation is very 
active. I find that under these circumstances one can keep the cell in 
an active condition, in an atmosphere of hydrogen, for a whole day 
when exposed to light. If light be at any time excluded, by means 
of the darkened box already mentioned, the movement becomes slow 
in about twenty minutes, and in half an hour usually, though not 
always, ceases. On re-exposure to light, rotation rapidly begins again, 
and in most cases completely recovers its old rate in about one or 
two minutes. If the cell be now darkened once more, a slowing 
down, or even complete arrest, of the protoplasmic rotation is effected 
in five to seven minutes. This experiment I have repeated a large 
number of times, and it may be safely shown as a class-demonstration. 
A longer re-exposure to light necessitates a more prolonged continu- 
ance of darkness in order to reproduce the quiescent condition. 
The experiments with Nitella were further checked by similar ones, 
carried on simultaneously with Elodea , as detailed above, and the 
differences are, I think, susceptible of an easy explanation. 
The cell of Nitella is a very large one, and consequently the volume 
of its cell-sap is relatively (as well as absolutely) greater than that in 
Elodea as compared with the amount of protoplasmic substance. 
Hence, supposing the cell-contents to be saturated to an equal extent 
with oxygen, it is clear that Nitella would be in a more favourable 
position when the further supply of oxygen was cut off than is Elodea. 
We might expect then, assuming the cells to be similar in other 
respects, that the movement of the protoplasm in Nitella would 
continue after that in the other plant had come to rest, and this is 
found to be the case. But, on the whole, the experiments seem also 
to show that Nitella can do with less oxygen than Elodea , for the 
difference in relative bulk in the two cases is hardly sufficient to 
xxxviii. 1887. A critical abstract of this paper is given by Professor Vines in 
Annals of Botany, Vol. i, p. 37 1 et seq. 
