224 DISCOVERY REPORTS 



Under the heading "adventitious", I include those forms obtained from melted ice. 

 Unfortunately, as insufficient material was examined, important comparisons with the 

 ice flora of the Arctic, which has been investigated thoroughly by Gran (1900 b), could 

 not be made. One outstanding fact, however, emerged, that is that the characteristic 

 ice species of the Arctic are not present in the Antarctic ice flora. Gran recorded nearly 

 thirty species of naviculoid diatoms and stated that they were represented abundantly 

 in the polar ice and also that Melosira hyperborea (Grunow) Van Heurck was very 

 characteristic of the ice floes in the Polar Sea, common in all samples, and predominant. 

 An entirely different ice flora was discovered in the samples examined. Fragilaria curta 

 and F. linearis were the predominant species, while Navicula corymbosa and Amphiprora 

 Oestrupii were observed in small numbers only. These latter species are common in 

 waters of the northern hemisphere, and were mentioned by Grunow as being found in 

 the Arctic Sea. Several of the other species were characteristically holoplanktonic, and 

 were probably of an entirely different origin from that of the species mentioned above. 

 The predominant species were truly neritic, and no evidence was obtained that would 

 suggest that the ice contained any freshwater species from the mainland. 



B. Neritic. 



Owing to the fact that the net touched bottom at several of the stations in the Brans- 

 field Strait, this flora proved to be a rich one, particularly in the case of sections (b) and 

 (c). Of the holoplanktonic section of this group it must be pointed out that although the 

 discoid diatoms were represented by many different species they were very few in 

 number as compared with the solenoid forms. Of the most interesting discoids, mention 

 must be made of Karsten's Coscinodiscus bouvet, one of the only forms of that genus to 

 adopt the chain-forming habit. This is a characteristic Antarctic species and found often 

 in considerable numbers. It was found associating with several species of Coscinodiscus 

 which are well known in northern waters. 



Species of Nitzschia were very abundant, also certain species of Chaetoceros, but by 

 far the most common form was Corethron. It is a matter of great difficulty to recognize 

 the essential difference between oceanic and neritic Corethron. 



Sections (b) and (c) of the neritic flora were observed chiefly in the material obtained 

 from the Bransfield Strait. St. WS 481 yielded a particularly rich flora, which included 

 several species characteristic of the Southern Ocean. 



EFFECT OF ENVIRONMENT ON FORM 

 A careful study of the forms found in each flora indicates that there exists a relation 

 between structure, and the chemical and physical constants of the water that supports 

 the population. The word "structure" is here used to indicate the size and form of the 

 frustule and not specific orientation. It is found that there is a correlation between the 

 internal cubical capacity and the external or surface area of a diatom frustule for a given 

 set of physical conditions. Broadly speaking it may be said that a warm-water diatom 

 flora is one whose individuals, under a given set of chemical and physical conditions, 

 seek to obtain maximum cubical capacity with a minimum of surface area, whereas a 



