444 DISCOVERY REPORTS 



many classes of marine animals. The latter almost invariably provide large individuals 

 of the same species in colder waters. It is in warm seas that the volume of marine 

 plankton diatoms appears to be greatest, with consequent reduction of the surface 

 volume ratio. Wimpenny's work on R. styliformis in the North Sea shows that this rela- 

 tion holds true for that species with great consistency over the temperature range he was 

 able to investigate. 



It will be realized that the size/temperature correlation we have been able to trace in 

 R. curvata agrees perfectly with this theory, until a point near the extreme upper limit 

 of the organism's temperature range is reached, when the size again decreases. Smaller 

 individuals of R. curvata undoubtedly predominated towards the colder limit of its 

 range, where I believe that auxospore formation is impossible for this species. 



The relation is particularly easy to visualize in terms of almost perfectly cylindrical 

 solenoid diatoms. If one considers two cylinders of equal volume, one twice the length 

 of the other, a simple calculation will show that the diameter of the long thin one is only 



— times smaller than that of the stouter individual. The surface area of the longer 



V2 



cylinder will however be greater than that of the stouter one in the proportion of 

 V2 : i. In nature the more slender individuals of the species studied are never long 

 enough to equal the stouter ones in volume, so that the increase in surface/volume ratio 

 will be even more marked. This seems highly significant in view of Hendey's conclusion 

 that cold water marine plankton diatoms in general tend to develop an elongated tubular 



habit. 



It appears possible that this tendency— increased surface area at colder temperatures, 

 and conversely decreased surface area at warmer temperatures— will be found to be 

 correlated with a necessity for maintaining a constant amount of metabolism within the 

 individual organism. It is obvious that so long as the surface/volume ratio remains the 

 same, the amount of metabolism will vary with the rate, i.e. directly with the tempera- 

 ture. It may well be that some such method of meeting increased rate of metabolism 

 is essential to warm-water species inhabiting media with a relatively poor supply of 

 nutrient materials. 



This, however, brings us to another factor which will probably be found to have a 

 complicating influence on the size/temperature relations of marine plankton diatoms — 

 nutrient ion concentration. I have not been able to follow it up in the present work, as 

 data relating to nutrient salts are not available at a sufficient number of the stations at 

 which R. curvata was found, and being a rare species, numerically speaking, it would 

 not be suitable for such a study. It seems to me to be very significant, however, that all 

 the localities where large diatom species have been recorded from the tropics are near 

 known centres of upwelling, and consequent relatively high nutrient ion concentration. 

 Recent work of Peters (1932) has shown that Ceratia reach their size maxima in these 

 areas, but as they are an essentially warm water genus, it may be unwise to stress unduly 

 the similarity of their size distribution to that of plankton diatoms. 



If the main hypothesis concerning the distribution of Rhisosolenia curvata examined 



