388 OBSERVATIONS ON PLANT PLANKTON. 



preservative employed was a 0-5 per cent, aqueous solution of 

 chromic acid; good results have also been obtained from Fleming's 

 solution and from platinic chloride of various strengths. I carried 

 tubes half filled with the chromic acid solution, which is about the 

 same density as ordinary sea-water. The diatoms, &c., were turned 

 out of the tow-net into a glass-jar, and allowed to settle for some 

 time. They were then collected from the bottom by means of a 

 dipping-tube, and added to the chromic acid solution. Several 

 times during summer, while working in haste and having large 

 quantities of diatoms to deal with, I removed some en masse with a 

 spoon from the tail of the net to the chromic acid solution. The 

 addition of a mass of diatoms, without the proper proportion of 

 sea-water, proved to be a mistake, since the contents of the cells 

 were disorganised in these cases. 



Distribution. — The seasonal occurrence of diatoms in vast 



numbers in the sea has loug been known, but no observations have 



been made that afford a satisfactory explanation of the causes that 



contribute to the phenomenon. On both the east and west coast, 



during the first months of the year, there is a remarkable prevalence 



of diatom life. We have no knowledge of the distance out in the 



Atlantic to which this extends, but in the coastal waters at this 



season the abundance of diatoms is extraordinary. Towards the 



end of March and beginning of April, the quantity diminishes, and 



for the rest of the year diatoms remain at a fairly constant quantity, 



except for here and there the occurrence of definite local banks of 



them. Peridinied, especially the species of Ceratiuni, are very few 



in number during the maximum period of diatom life, but they 



come upon the scene with the waning of the diatom season, reach 



a maximum about August, and linger on in fair quantities until 



December at least. Early in April, during four or five days' work 



in Loch Fyne, when diatoms were exceedingly plentiful, I succeeded 



in obtaining only one specimen of Cemtiiun Tripos, which, at other 



seasons, especially summer, is the predominant plant organism in 



the western lochs. During the period of most active diatom life, 



the surface of the sea is almost monopolised by a diatom Skeletonema 



costatum; while species of Coscinodisciis, Biddulphia, Ditylum, Blrizo- 



solenia, &c., abound in the layers beneath, the greatest quantity of 



these being generally obtained at 5 fathoms, very few below 25 



fathoms. The surface of the Clyde sea-area so teemed with 



Skeletonema in April that a tow-net could be one-third filled in a 



few minutes with a scum consisting of it and other diatoms in 



smaller quantity. When I revisited the same stations in the end 



of July, I could not obtain a single Skeletonema, and its absence (at 



all depths) continued to puzzle me during my examination of the 



west coast in August, except for a few specimens obtained near 



Oban, and its remarkable occurrence in this, Loch Etive. While 



(with this one exception) it could not be found outside, the surface 



of Loch Etive simply swarmed with Skeletonema in abundance, equal 



to if not greater than that of the Clyde sea-area in spring. While 



it occurred as a surface organism in spring, it was far more plentiful 



in Loch Etive at 5 fathoms than at the surface in summer. There 



