DIATOMACEM 



421 



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1 



Fig. 'i'^^ h.T-Ano^nceneis spherophora. a^ c, girdle- 

 view ; 6, valve-view. The endochrome-plates are 

 shaded ( x goo). (After Pfitzer.) 



or zonal view ; the aspect in which the surface of the. valve is turned to- 

 wards the observer is the side or valve view. In many diatoms the 

 central space on the valve view 



not occupied by transverse 

 strise shows at its middle and 

 at each end a strongly refractive 

 thickening known as a node or 

 nodule ; and these nodules are 

 connected with one another by 

 a longitudinal Ime or rib — the 

 raphe or suture. The primary 

 classification of the genera of di- 

 atoms usually adopted depends 

 on the presence or absence of 

 this raphe. 



Each diatom-cell contains a 

 nucleus and a nucleole. The 

 chlorophyll occurs in the form 

 of plates or bands arranged 

 with more or less symmetry, and there are usually also drops of oil, 

 especially when conjugation is about to take place. A very few di- 

 atoms are green; but in the great majority of cases the colour of the 

 chlorophyll is obscured by a 

 characteristic brown pigment 

 known as diatomin, readily 

 soluble in alcohol, forming a 

 brownish-yellow solution which 

 is only slightly or not at all 

 fluorescent. With concentrated 

 sulphuric acid it assumes a 

 beautiful blue-green colour. 

 Petit (Brebissonia, 1879-80, 

 p. 81) has very carefully in- 

 vestigated the chemical and 

 physical properties of the 

 colouring matter of diatoms. 

 He regards diatomin as a 

 compound of chlorophyll and 

 phycoxanthin, and as having 

 a great analogy with the chlo- 

 rophyll of the higher plants, the two spectra being very similar. 



Many of the solitary species of diatom, such as those belonging to 



Fig, 354 b.— 

 valves, side 



Gompkonenta constriciitni Ehrb. j, 

 view, showing nucleus : gi^ g,,^ girdle- 

 views ; y, transverse section through middle of cell, 

 showing siliciiied cell-wall, one half overlapping the 

 other ; ^ kf nucleus ; ^, dense protoplasm ; jf., £,„ 

 girdle sutfaces (magniiied). (After Pfitzer.) 



