4 6 



DISCOVERY REPORTS 



whereas in the deep-water stations (seven in daylight and seven in darkness) we have 

 the following proportions : 



We see that in the deep-water stations the proportion above and below 250 m. is nearly 

 the same in the two series for daylight and darkness. Taking the top 250 m. only, and 

 expressing the proportions at different depths as percentages, we get figures that com- 

 pare well with those for the shallow-water stations : 



These figures have been used in the illustration for comparison with the shallow- 

 water results and the proportions at the lower levels of the deep-water stations drawn 

 to the same scale. 



In the deep-water stations 39 per cent of the quantity present in the 250-100 m. 

 layer during the night have passed up into the photic zone above during daylight. In the 

 shallow-water stations the proportion is 42-3 per cent. Now we have already seen that 

 approximately 30 per cent of the specimens in this 250-100 m. layer are dead — i.e. lack 

 chromatophores — so that it would seem that another 30 per cent are either resting or 

 moribund and no longer moving upwards during daylight. On comparing the propor- 

 tions in the layers of 50-0 and 100-50 m. in daylight and darkness in each series it is 

 seen that the distance up and down travelled by individual diatoms must be at least 

 50 m. each way. 



If these figures are a true indication of a vertical migration, and it seems impossible to 

 interpret them otherwise, it would appear that this is the first record of a diurnal vertical 

 migration in the phytoplankton — a vertical migration which, it should be noted, is the 

 opposite to that of the plankton animals, which rise towards the surface at night and sink 

 in the daytime. Such an opposite vertical migration might have a distinct survival value 

 for these plants. It is perhaps significant that it has been discovered for Coscinodiscus, 

 a rounded form, which, if it is able to alter its specific gravity by the production of gas 

 or gases within the envelope, would be able to rise bubble-like through the water. 



C. centralis, Ehr., was only met with at two stations: WS 112, 6200 (62) and WS 114, 

 500 (5) on the C line at the end of May 1927. 



