11 



threads proceed from hollow spines (as in Thalassiosira*), which 

 contribute to the alga's powers of flotation. 



At the same time that these peculiarities in the construction 

 of the alga are favourable to it. in its floating state, so long 

 as the conditions of life are propitious, they weaken its powers of 

 resistance to variations in the physical and chemical condition of 

 the ocean. In the open sea the changes are slight, and take place 

 gradually; but, near the coast, much fresh water combines with 

 the sea water, and the temperature varies much more there with 

 the seasons than in the open sea in corresponding latitudes. 



As a protection against such variations, the Neritic Diatoms 

 are given their spores, which are formed towards the close of the 

 period of vegetation. 



The protoplasms of the cells withdraw from the hairs (in Chw- 

 toecros), and away from the walls of the cell, first on one side, 

 where a new, thicker, arched valve forms inside the old one, thus 

 dividing tbe original cell into two spaces, of which only one con- 

 tains the protoplasm. Subsequently, the protoplasm withdraws from 

 the other side of the original cell, and becomes enshrouded in a 

 thick, strong silicious shell. The spore is than fully formed, but 

 remains enclosed within the walls of the original cell until that falls 

 oft or dissolves. The substance of the cell has now become reduced 

 to halt or quarter of its former capacity, and the walls are about 

 three times as thick as the vegetal cell walls. The walls may 

 oiten he furnished with spines or other appendages, but these 

 spines are solid, not hollow like the hairs of the vegetal cells. 



fn the difl^erent families and species, the method of formation 

 01 the spores varies somewhat in the details, but in most of them it 

 takes place in the manner above described, and has already been 

 accurately defined by SciiiiTT [88] in respect to some Neritic Clice- 

 toceros species from the Bay of Kiel. 



As soon as a species commences to form spores it gradually 

 disappears from the Plankton, the spores sinking to the bottom as 

 indicated by Schutt [88]. They possess a much greater power of 

 resistance than the vegetal cells, as the difl'usion takes place less 

 rapidly through the thick cell walls. Thus, for instance, they are 

 much more slowly discoloured by alcohol than the vegetal cells. 



J- he spores have been known from the earliest period of dia- 



omic research. The diatoms have been mostly sought for in samples 



lom the bottom, in which the vegetal cell-walls of the Pelagic forms 



aie very seldom met with, whilst the spores are found somewhat 



frequently. 



Owing to this it is that Ehebnbee(} [45] has figured several 

 onus of spores, without however knowing their true character, 

 -•-he first species of the Chcetoeeros was but described from the 

 pores, whilst the accompanying vegetal cells were only dis- 

 covered many years afterwards (Chcetoeeros didymum, Ch. fur- 

 cellahmij. 



Spores within the vegetal cells, the so-called inner cells, 

 ^vere first described by Brightwell [56] in the Chcetoeeros Wighami, 

 subsequently by Lauder [64] in some species of Chcetoeeros obtained 



from Hong I-J, 



ong, and in the Baeteriastrum varians later by Castra- 



) A detailed account of the organic structure and various floating- ap- 

 pliances of the Plankton Diatoms is given by ScHiJTT [93]. 



CANE [79], Grunow [84], and by others. It was first Schxjtt who 

 subjected them to a detailed investigation. 



By the efforts of Cleve [94, 96, 1—2, 97] and Gean [97, 

 1 — 2] spores have been shown to exist in most of the Neritic 

 Diatoms found on our shores, and these are also being discovered 

 in species in which they were formerly unknown. We therefore 

 consider it likely that in the course of time they will be found in 

 all Neritic forms. 



Up to- the present they are known to exist in the following 

 28 species of the Norwegian coasts. 



Baeteriastrum varians 

 Biddulphia aurita 



— mobiliensis 

 Chcetoeeros cinctum 



— constrictum 



— contortum 



— coronatum 



— curvisetum 



— debile 



— diadema 



— didymum 



— furcellatum 



— gracile 



— laciniosum. 



— radians 



— Schuettii 



— scolopendra*) 



— seiracanthum 



— simile 



— soeiale 



— teres 



— Weissflogii 



— Wighami 

 Fragilaria oceanica 

 Melosira hyperhorea*) 

 Rhizosolenia setigera 

 Thalassiosira gravida 



— hycdina 



— Nordenskioldii 



The Norwegian Neritic Diatoms in which the spores are not 

 yet known, consist of the following only, apart from the true rarities: 



Cerataulina Bergonii Lauderia annulata 



ChcBtoeeros crinitum Leptocylindrus danicus 



— Willei Podosira glaeialis 



Coseinodiscus polychordus SMletonema costatum 



Ditylum Brightwelli Thalassiosira gelatinosa 



Eucampia Zoodiacus (== Coseinodiscus exentricus v. 

 Guinardia flaccida gelatinosa). 



Most of these are rare species, we having met with but three 

 in any great number (Guinardia flaccida, Leptocylindrus danicus, 

 SMetonema costatum). It may, however, occasionially be difficult 

 to determine whether a species is oceanic or neritic, especially if 



*) According to ne-w, and still unpublished obserVTiVi »t<tby Gbah. 



