Physiology of the Purple Sulphur Bacteria. 9 
unfavourable, growth was rather slow. The chief results were as 
follows:— 
1. In the cultures of the first two series at most only a slight 
growth was observed, 1 even after the lapse of four months. 
2. In the cultures of series (3) and (4) a moderate develop¬ 
ment took place after one month only in the control solutions, 
without organic substances. A fortnight later the development in the 
control solutions was complete and vigorous, a slight development 
was observed in dextrose and maltose, and a very slight develop¬ 
ment in mannite. After four months the growth in dextrose, 
maltose, mannite and potassium formate was good, or very good : 
long before this the zoogloete of the control cultures were fully 
developed and had commenced to show signs of dying. 
3. The growth is constantly better and quicker in the solutions 
with the lower concentration of organic substance. 
A few other combinations were tried :— 
Glycocoll + calcium formate (0'2%). 
Glycocoll -f- calcium lactate (0’2%). 
Asparagine mannite + (0-2%). 
Urea + mannite (0-2%). 
Peptone + mannite (0-2%). 
» + glycerol (0-2%). 
„ + dextrose (0‘2%). 
And a decoction of Enteromorpha. Only in this last was a slight 
growth observed. 
It would appear then that all these substances are without 
favourable influence on the development of the sulphur bacteria: 
rather would it seem that they tend to inhibit that development. 
It seems remarkable that precisely the most highly organised 
compounds permit of the best growth. It must be noted, however, 
that the growth is never so good as when only mineral substances 
are present, that it only takes place if these mineral substances are 
added, that the maximum of development occurs about two months 
later than in pure mineral solutions, and that the lower the 
concentration the better is the growth. Perhaps it may be 
assumed that precisely these substances will, in mixed cultures, 
be most rapidly attacked by other micro-organisms, and decomposed 
so far as to destroy their inhibiting influence. 
If now it be the case that the purple sulphur bacteria grow 
best in solutions containing no organic compounds, it might well 
1 Only with calcium lactate was the development more than very slight. 
