I 20 
Harriette Chick 
other Fern-like plants of its period, it had crossed the Spermophytic 
frontier, so that its sporangia described above were in reality of 
the nature of pollen-sacs. 
All the sections were prepared by Mr. Lomax, of Bolton. 
I am indebted to Mr. L. A. Boodle, F.L.S. for the photograph 
reproduced in Fig. 1, and to my wife, Mrs. D. H. Scott, F.L.S., for 
the sketches in Fig. 2, a —c. 
THE BIOLOGICAL LIMITATIONS OF THE METHOD 
OF PURE CULTURE. 
rnHE use of pure cultural methods in the study of the lower 
organisms has been a late development, but it has been 
adopted with an eagerness and practised with a faith that is not 
surprising when one reflects that all recent advance in certain 
sections of biology has been rendered possible by its means. When 
bacteria, for example, first became objects of special study, progress 
was slow and uncertain, and it became evident that some such 
method was absolutely essential. Previous to the use of plate- 
cultures with solid media by Robert Koch, a pure culture had 
rarely been obtained ; afterwards the science of bacteriology 
progressed by leaps and bounds, for Koch’s method furnished a 
means, simple as it was efficient, whereby single individuals in a 
mixture of bacteria could be separated and kept apart, while each 
grew and reproduced independently. Colonies were thus formed 
which consisted of one species alone and which were large 
enough to be successfully manipulated and propagated further in 
pure culture. 
It is surprising that a similar method should not have been 
already practised more extensively in the study of other groups. 
In the case of the lower green algae, for example, great confusion 
has existed as to the proper identity and morphological relations 
of many forms. To attack such problems there was no need to 
wait for such an epoch-making discovery as that of Robert Koch, 
for the aim of the worker would have been merely to trace the life 
history of the species in question apart from other forms with 
which it might be confounded morphologically; in such cases a 
strictly pure culture is not necessary or desirable, and cultures 
sufficiently pure for the purpose can obtained by comparatively 
simple means. In the case of bacteria, on the other hand, a class 
