324 BOTANICAL GAZETTE [NOVEMBER 
solid culture medium. ‘These solid cultures were made in Petri dishes. 
Although such dishes have the same advantage as Stender dishes 
in that they can be put under the microscope and the cultures studied 
without disturbance, they cannot safely be employed in an ordinary 
laboratory for more than very brief periods at a time. The air of 
the ordinary laboratory is excessively dry (2). In consequence, there 
will be a quite too rapid and altogether excessive concentration of 
the culture medium in Petri dishes, even if covered, in all cases in 
which experiments last for more than a few days. In our climate, 
the air in the laboratory in which we worked is more humid during 
the winter (the rainy season) than in most laboratories. We therefore 
ventured to continue these cultures for some time. In summer and 
autumn here, and at all seasons in most laboratories elsewhere, Petri 
dishes are unsuitable for the culture of algae. They should be 
replaced by flasks holding a much larger volume of the moist culture 
medium and furnished with narrow necks closed by very tightly 
rolled cotton plugs. At certain seasons it would be well to cover 
the plugs with the rubber caps made for that purpose. : 
All dishes and solutions were carefully sterilized before the algae 
were sown in them. Instruments were sterilized immediately before 
use. In this way few if any organisms were added from outside 
when fresh cultures were made; but as, in order to save time, we 
started with small masses of material rather than with single cells 0". 
single filaments, our cultures were not pure. We experienced v 
siderable annoyance in a few instances from the wth of panes 
or of algae other than the ones especially sought. Nevertheless, : 
such experiments as we had in mind, the extreme pains and the a 
time required to obtain pure cultures would hardly be justified 
results. Furthermore, the behavior of living organisms eee a 
cultures may be due in part to two unnatural, that is to say WN"? 
factors, namely, the artificial culture medium and the vessel ° sk 
it is contained, and the freedom from the competition with we" a 
of organisms and their products. Our aim was, eee ae “a 
our algae under as nearly as possible natural conditions, | a — 
realize that the degree of naturalness which we attained 1s y = : 
stage in advance of the frank artificiality of pure culture. — of ret ‘ 
One of the greatest difficulties encountered in the culture : 7. 
