392 E. J. ALLEN. 
sium chloride might be doubled. Similarly the Magnesium suiphate 
could be omitted or doubled without detriment. In the case of the 
Calcium chloride, also, the amount of permissible variation was very 
great, though not quite so great as with the Potassium chloride and 
Magnesium sulphate. These results were a little surprising, as it had 
generally been assumed that the Plankton diatoms were particularly 
sensitive to changes in the composition of the medium in which they live. 
It is clear, however, that provided all the essential substances are present, 
in some cases even excessively minute quantities being sufficient, the 
composition of the medium can be very greatly altered without affecting 
the organisms. 
Incidentally, the facts which I have been describing would appear to 
have an important bearing upon all theoretical questions concerning the 
relation of the organism to its environment. If such a minute change in 
the environment as is involved in the addition of 1 per cent of natural 
sea-water to the artificial solutions can make all the difference between 
reproduction and no reproduction of the diatom, can we be said in the 
case of any organism whatever to have an adequate conception of what 
the effective environment really is? Clearly we are very far imdeed 
from appreciating or understanding how intricate and varied are the 
many factors upon which the life of even the simplest plant or animal 
depends. 
But this is a digression. Let us return to the diatom cultures, and 
ask what is the chemical nature of the essential substance, a trace of 
which must be present in the artificial culture solution before the diatoms 
can flourish. Up to the present I have been unable to give any final 
or adequate answer to this question. Certain hints and suggestions have, 
however, been obtained which give us an indication of the lines upon 
which the answer to the question is to be sought. Some of the experi- 
ments which have given rise to these suggestions may be of interest to 
you. 
To a flask containing some of the artificial sea-water, which had been 
treated with nitrate, phosphate and iron, a small fragment of the green 
seaweed Ulva was added and the liquid boiled for a few minutes. A 
sheht infusion of seaweed in the artificial sea-water was thus made. 
The piece of Ulva was removed and the liquid allowed to cool. It was 
found that good cultures of the diatom could then be made in the solu- 
tion. The slight infusion of vegetable matter therefore performed the 
same function as the 1 per cent of natural sea-water, which had been 
added in the previous successful experiments. 
When, on the other hand, instead of making an infusion of the seaweed 
