1897] POLYMORPHISM IN THE ALGAE ig 
to solve the problem of the independence or genetic connexion of 
two or more algae with the greatest possible certainty? In my 
view such an investigation demands attention to three important 
points :— 
(1) The pure cultivation of the organisms in question 
Just as a pure culture is obviously essential for any research 
into the history of development of a fungus or of a bacterium, a 
similar culture is the necessary starting point in the investigation 
of the life-history of a lower alga. In general, the algae grow 
much more slowly than the fungi and bacteria; cultures, therefore, 
have to be maintained for a longer period, and the chance of the 
accidental introduction of foreign forms into an originally pure 
culture must not be neglected. Minute cells or spores of Proto- 
coccoideae, etc., are present in the dust of the air. It is only 
necessary to leave a sterilised solution of nutritive salts, not very 
well protected, in the light, in order to convince oneself that algae 
get into it with the dust. It is clear from the works of Chodat and 
Borzi, that these workers had only impure cultures at their disposal, 
since, on the one hand, they used material taken direct from its 
-natural habitat, and containing numerous species of algae, and on 
the other, they paid no attention to the sources of error arising from 
the exposure of their cultures to dust. 
(2) Direct observation 
In the case of the lower algae it is always necessary to observe 
directly under the microscope, the course of development or the 
transformation of one form of cell into another. In default of a 
pure culture, this method may, under certain circumstances, do 
instead ; but it should be used in any case, even if the culture is to 
all appearance pure. A combination of the two methods leads to 
very certain results. Chodat and Borzi have employed them far 
too little. For instance, Borzi ought to have isolated the double 
zoospores of Prasiola, etc., and then uninterruptedly observed their 
subsequent fate, in order to convince himself that they actually 
turned into Raphidium. And similarly it would be necessary to 
observe directly the development of the filamentous alga, whether 
Prasiola or Protoderma, from Raphidiwm, a thing which it appears 
Borzi never really saw. The same criticism holds in regard to 
Chodat’s statement that Plewrococcus vulgaris changes into Stigeo- 
clonium. The immediate transformation of.an undoubted Plewrococcus 
cell into a Stigeocloniwm has not been seen, any more than a trans- 
formation of the latter into Plewrococcus, 
