1897] POLYMORPHISM IN THE ALGAE 111 



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 

 set 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 Baphidium. And similarly it would be necessary to 

 observe directly the development of the filamentous alga, whether 

 Prasiola or Protoderma, from Baphidium, a thing which it appears 

 Borzi never really saw. The same criticism holds in regard to 

 Chodat's statement that Pleurococcus vulgaris changes into Stigeo- 

 clonium. The immediate transformation of an undoubted Pleurococcus 

 cell into a Stigeoclonium has not been seen, any more than a trans- 

 formation of the latter into Pleurococcus. 



