228 



DISCOVERY REPORTS 



It would be very difficult to say how the chemical and physical factors of the water 

 exert the influences that bring about these modifications. It is likely that the density of 

 the water, which as shown in the table on p. 159 varies considerably from the tropics to 

 the polar regions, might be responsible to a large degree. 



1 

 V J 





Fi g- 3- 



ANALYTICAL KEY TO THE GENERA 

 The construction of a key is a matter of great difficulty in diatoms, because of the 

 difficulty of finding adequate expression for the definitive characters, to make them com- 

 prehensible to the uninitiated, and because the size of the organisms does not allow their 

 various aspects to be examined with ease. The following key is mainly for the field 

 worker, based upon characters observed in fresh material or in material carefully pre- 

 served in formalin. 



Great care must be taken in consulting section 1 of the key. In section 1 are included 

 all the main divisions of the diatoms observed in the Antarctic. Section 1 is necessarily 

 large, as the old division of diatoms into two groups, viz. Centricae and Pennatae, which 

 is wholly inadequate in the case of plankton diatoms, has been abandoned, and the first 

 section has been framed upon altogether different lines to include the genera of zygo- 

 morphic diatoms which form such a large part of the phytoplankton. Correct interpre- 

 tation of the characters set out in section 1 will take the worker directly to the group of 



