12 



it only rarely appears, and then iu small numbers. It is, therefore, 

 not improbable that some of the latter species which have been 

 denoted above, will, on further investigations being made, prove 

 to be oceanic (for instance Guinardia). 



The spores are also known to exist in a number of fresh 

 water forms. In the Plankton Diatoms of lakes, they have only 

 been known of later years (Lauteeboen [96], B. Schboxiee [97]), 

 as these algae themselves were only discovered a few years ago. 

 But in bottom-forms, and especially in those species which in- 

 habit ponds and pools, they have long been known and spoken 

 of under various names such as, inner cells, gemmae "Crati- 

 cular-Zustande"; see Ppitzee [71]. Other freshwater algae 

 have also similar cysts as the diatoms. 



In the freshwater forms they appear, especially when the ponds 

 dry up. The algae, by their aid, repose in the dry mud until they 

 again obtain favourable conditions for their development. This 

 power of resting, which is a necessary condition for populating the 

 pools with life, is also met with in animals of various kinds; cru- 

 stacese, rotatories, and other dwellers of the ponds can, like the 

 algae, form encysted resting stadia. 



In these organisms the spores chiefly serve the purpose of 

 preventing them from drying up. The outer conditions which 

 lead to the formation of spores in the Marine Plankton are still 

 unknown. Neither does one as yet know anything concerning their 

 sprouting, or the conditions under which such takes place. 



The physical and chemical factors which, it may be presumed, 

 are of importance to the life of the algae, are light, warmth, and 

 the chemical constitution of the sea water. All these factors vary 

 most in the upper layers of water, whilst the conditions as a rule 

 become more uniform the deeper one descends. 



If now the spores, as we presume, are of any consequence to 

 the continued existence of the species, then the diatoms must be 

 not only dependent on the hydrographical conditions of the surface 

 layers in which they vegetate, but the hydrographical conditions at 

 the bottom must also be of such a nature that the spores may 

 periodically rest, or be roused into sprouting. If a rich Neritic 

 Flora is to have a chance of developing itself, and he annually 

 renewed from, spores at the bottom, the latter must not lie beyond 

 such a depth as to allow of the yearly variations in light, tem- 

 perature &c. reaching it. 



Our deep Norwegian fjords, which are constantly dark at the 

 bottom, and where the bottom-temperature throughout the year can 

 only vary from 1" to 2" degrees, cannot be expected to contain 

 any stationary Neritic Plankton. On the other hand, in shallow 

 fiords or coastal waters, the light can penetrate, and the annual 

 periods of temperature can be perceived, more or less perceptibly, 

 down to the bottom itself. There during summer a rich flora 

 of southern forms can be roused to life, which rest during the 

 winter, whilst, on the other hand, during that season, the northern 

 species can meet with so low a temperature as to allow of their 

 growing up. 



From this point of view we will compare the Neritic Flora of 

 the various northern coastal waters, which hitherto has been best 

 known, and endeavour to shew that the true conditions correspond 

 exactly to our assumption of the importance of the bottom. 



The typical specimen of a shallow fjord is Lim Fjord, whose 

 remarkable plankton-conditions have been described by Dr. C. G. 

 Job.. Peteesen in his lately published work [98]. 



The depth of the fjord averages 3 to 4 fathoms, and the hy- 

 drographical conditions may be regarded as uni(iue from the sur- 

 face to the bottom. 



The temperature may vary from O-.S" to 18".4 Celsius. Ac- 

 cording to Peteesen's investigations, the Plankton is excessively 

 abundant for most part of the year, and notwithstanding that the 

 water in the Lim Fjord is in constant movement from the Noith 

 Sea to the Cattegat, it nevertheless possesses a Plankton Flora, 

 which both in quality and (juantity is sharply defined from that 

 of the North Sea, and which is, in the main, different from that 

 of the Cattegat. 



The Lim Fjord also possesses a local, stationary Diatom-Plank- 

 ton which changes in composition with the seasons, but which proves 

 to be quite different from the Plankton in the neighbouring seas at 

 all times of the year, as is proved by Peteesen who carried out 

 three series of continuous observations throughout the Lim Fjord 

 from the North Sea to the Cattegat (October 96, April and July 97). 

 The various species succeed each other with the seasons. In 

 December and January, the Plankton is very scarce; in February, 

 and especially in March, the winter forms (Biddidphia aurita, 

 Lauderia cystifera) appear; in April these are succeeded by the 

 spring forms, which in that inonth bring the amount of Plankton 

 to a maximum. In May and June, the Plankton is somewhat less, 

 but during the summer months of July^ September the fjord was 

 again filled with great quantities of diatoms, especially Chcetoceros debile. 

 All tliose species which cliaracterise the Lim Fjord Plankton 

 are Neritic forms which produce spores. Peteesen concludes that 

 rich Diatomic Plankton must originate in the Limfjord itself from 

 the interaction of the bottom and the waters that flow in to the 

 JJjord. Of what this interaction consists he cannot express any 

 decided opinion. 



It ajipears to be his idea that the great development of the 

 Plankton Flora in the Lim Fjord is due to the exceptionally fav- 

 ourable conditions that are present there, as a greater amount of 

 nitrogenous compounds are conveyed to its waters tlian is to be 

 met with in tlie open sea, or deep fjords. 



This idea, which in connection with other matters, was first 

 promulgated by Hensen, and subsequently by Steoxitmann [96], 

 has been adopted in respect to the Plankton in lakes, seems to be 

 well founded, when it is remembered that the open sea is highly 

 deficient in nitrogenous compounds. 



In the mud of shallow fjords a great mass of organic sub- 

 stances are always present, which, gradually rotting, give off nitro- 

 gen to the water. 



This factor, however, only becomes of consequence provided 

 germs are present which can utilise the favourable conditions of 

 nourishment. One may imagine that the diatoms enter the fjord 

 from without, and the oceanic species, which especially in May and 

 June appear in quantities, ai'e probably introduced by the in-flowing 

 curr nts. 



But the Plankton of the Lim Fjord chiefly consists of Neritic 

 forms, and as the Neritic Flora of the fjord is different from that 



