OBSERVATIONS ON PLANT PLANKTON. 389 



was no local, low temperature condition that would serve to explain 

 this, Loch Etive being, in point of fact, nearly two degrees warmer 

 than the sea outside. In its greater abundance at 5 fathoms than 

 at the surface it agrees with all other diatoms during the summer 

 and winter, — at all seasons, in fact, except during the period of 

 maximum occurrence. If one sets nets at the surface, 5 fathoms, 

 10 fathoms, and 20 fathoms, the result will be the greatest capture 

 of diatoms in general at 5 fathoms, a nearly equal (smaller) quantity 

 (often of the very same organisms) at the surface, and at 10 fathoms, 

 and a much slighter quantity at 20 fathoms. Excluding March 

 and April, I found no notable exception to this rule, except in 

 several hauls in December, in the Dunoon basin, when I obtained 

 more from 10 than from 5 fathoms. From the character of the 

 special pigment diatomine, which masks the chlorophyll in diatoms, 

 and its resemblance to the corresponding pigment in Flueophijcea;, 

 one would expect this vertical distribution of diatom life, since its 

 maximum occurrence is nearly at the same depth beneath the sur- 

 face as the Laminarian zone. It is plain that free-floating organisms 

 like diatoms, from being subject to currents and immersions of 

 layers of water, must be liable and adaptable to certain changes of 

 depth, and to the varying quantity and quality of sunlight that 

 reaches them at such depths, just as shore algae are subject to such 

 periodical changes in the rise and fall of the tides. 



In March to April, the plant plankton of the east coast, judging 

 by a hurried examination of it in bad weather, closely resembled 

 that of the west coast outside the Clyde sea-area. The character- 

 istic diatoms were, in the order of their abundance, Coscuwdisciis 

 concinnus, Chatoceros boreah's, Ditijhim Brujhtwellii, Ch(Eloceros deci- 

 piens, Rhizosolenia Shrubsulii, Skeletonema costatum, and Coscmodiscus 

 radiatus ; while, of other organisms, Ceratiwn Tripos, in places here 

 and there, almost rivalled Ditijlum, but generally was about equal 

 to Rhizosolenia. On entering the Clyde sea-area, however, Ceratium 

 became much more scarce ; and in Loch Fyne, as said above, only 

 one specimen was obtained in several days' tow-netting. On the 

 other hand, Skeletonema, from being scarce, at once leapt easily into 

 first place, far exceeding any other Clyde diatom ; Coscinodiscus 

 concinnus also became far more plentiful ; and both species of 

 Chatoceros, named above, also increased greatly. There were 

 numbers of other diatoms, now present, now absent, but these were 

 the forms characteristic of the season. In summer, while diatom 

 life was scarcer, there was a much greater variety of forms ; but 

 ouly in the Sound of Mull and in Loch Etive an approach to the 

 quantities obtained in the Clyde in spring. I sent nine typical 

 tubes to Professor Cleve, who has kindly favoured me with the 

 report on them in the accompanying table. 



These diatoms and their distribution were fairly typical of all 

 the localities, now one form, now another, varying in abundance; 

 but Chcctoceros curvisetiis, C. decipiens, C. didijmus, Rhizosolenia 

 Shrubsolii, Coscinodiscus concinnus, TCucampia zodiacus, and Stephano- 

 pyxis Turris being the prevalent forms. Professor Cleve noted that 

 the fronds of C. curvisetus were spore-bearing, — a fact which I had 



