GENERAL NOTES 225 



cold-water diatom flora, existing under an entirely different set of conditions, seeks to 

 obtain maximum surface area for a minimum cubical capacity in its individuals. 



The poorness of the waters of the tropical and subtropical Zones in silica and 

 nutrient salts, together with high hydrogen-ion concentration, gives rise to populations 

 of thin-walled diatoms whose frustules have a relatively large internal cubical capacity 

 and a relatively low surface area. As previously mentioned, various structural devices 

 are exploited to increase the internal capacity in certain forms, e.g. the levigated 

 discoids. Indeed, there is a marked tendency in most tropical genera to approximate to 

 a spherical form, as a sphere provides the maximum internal capacity for the minimum 

 surface area. This is accomplished in some species by the formation of complex annular 

 girdles, and a deepening of the valve mantle. This endeavour to obtain a maximum 

 internal capacity with a minimum surface area is to provide volume for metabolic 

 products such as oil and possibly gases, which lighten the large frustules so characteristic 

 of the tropical flora and allow them to maintain their position in the surface layers of 

 the ocean. 



Turning to the flora supported by the colder waters, it is found to be made up of 

 species which produce small but rather more vigorous frustules, as nutrient salts in the 

 higher southern latitudes never fall so low as to act as a limiting factor to growth. While 

 the intense cold of the extreme south does not favour the formation of large and luxuriant 

 individuals, the population existing within the Antarctic Zone indicates that definite 

 modifications in size and structure are made to accommodate it to the change in en- 

 vironmental conditions. Thus the decrease in the size of the frustules of cold-water 

 diatoms, probably brought about by decreased sunlight, together with heavier silici- 

 fication, produces a population of greater density, that is, a population in which 

 there is greater mass per unit volume of diatoms. It is this increased density of the 

 individuals that makes it necessary for the surface area of the individuals to be 

 increased if the plankton is to be maintained in the photic layers. This increase in the 

 surface area is obtained by adopting the tubular formation of the frustule and the de- 

 velopment of large appendages. 



Among the smaller forms, members of the genus Chaetoceros occur abundantly in 

 each of the two floras, but again the same general rule holds good. Tropical species of 

 Chaetoceros are relatively large in the main body of the frustule and the bristles are short 

 and thin, while in the Antarctic species the body of the frustule is reduced to a minimum, 

 and the bristles are very long and pendulous and frequently bear spines. It is essential 

 that the individuals of both floras possess the power of maintaining their position in the 

 surface layers of the water in order to avail themselves of the sunlight. In some genera, 

 e.g. Planktoniella and Chaetoceros, it is probable that the diatoms possess the faculty of 

 exercising some control over their own buoyancy and so regulate their position in the 

 photic layer. 



Careful survey of the two floras indicates that this correlation of internal capacity and 

 surface area plays an important part in the size modification of the species, particularly 

 in those which are truly holoplanktonic. 



