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 Chce- 

 toceros), 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 the 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 

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

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

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

 often be furnished with spines or other appendages, but these 

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



In the different families and species, the method of formation 

 of 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 SCHUTT [88] in respect to some Neritic Chce- 

 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 diffusion takes place less 

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

 much more slowly discoloured by alcohol than the vegetal cells. 



The spores have been known from the earliest period of dia- 

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

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

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

 frequently. 



Owing to this it is that EHEENBEKG [45] has figured several 

 forms of spores, without however knowing their true character. 

 The first species of the Chcetoceros was but described from the 

 spores, whilst the accompanying vegetal cells were only dis- 

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

 cellatum). 



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

 were first described by BHIGHTWELL [56] in the Chcetoceros Wighami, 

 subsequently by LAUDEB [64] in some species of Chcetoceros obtained 

 from Hong Kong, and in the Bacteriastrum varians later by CASTRA- 



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

 pliances of the Plankton Diatoms i> given by SCHUTT [93]. 



CANE [79], GRUNOW [84], and by others. It was first SCHUTT who 

 subjected them to a detailed investigation. 



By the efforts of CLEVE [94, 96, 12, 97] and GRAN [97, 

 12] 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 iound in 

 all Neritic forms. 



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

 28 species of the Norwegian coasts. 



Bacteriastrum varians 

 Biddulphia aurita 



mobiliensis 

 Chcetoceros cinctum 



constrictum 

 contortum 

 coronatum 

 curvisetum 

 debile 

 diadema 

 didymum 

 furcellatum 

 gracile 

 laciniosum 

 radians 

 Schuettii 

 scolopendra*) 

 seiracanthum 

 simile 

 sociale 

 teres 



Weissflogii 

 Wighami 



Fragilaria oceanica 

 Melosira hyperborea*) 

 Rhizosolenia setigera 

 Thalassiosira gravida 

 hyalina 

 NordensTcioldii 



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 



Chcetoceros crinitum Leptocylindrus danicus 



Willei Podosira glacialis 



Coscinodiscus polychordus SJceletonema costatum 



Ditylum Brightwelli Thalassiosira gelatinosa 



Eucampia Zoodiacus (= Coscinodiscus 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, 

 STteletonema costatum). It may, however, occasionally be difficult 

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



*) According to new, and still unpublished observation* by GKAK. 



