96 



Bacillariede 



Plate-like chromatophores occur in nearly all the naviculuid diatoms, and 



they have approximately the same form and 

 disposition in groups of allied species. In most 

 of the centric diatoms the chromatophores take 

 the form of rounded or lobed discs arid are 

 more or less numerous in each cell (fig. 69 B 

 and C). 



They are of a yellow-brown or golden-brown 

 colour, although in a few diatoms, such as 

 Navicula viridis and N. cuspidata, they are 

 occasionally green. They contain chlorophyll, 

 but this is masked by a brown colour often 

 stated to be due to a pigment which has been 

 termed ' diatomin.' Recent investigations, 

 however, into the colouring matters present 

 in chromatophores, not only of diatoms, but of 

 other algae and also of higher plants, have 

 shown that many misconceptions have existed 

 with regard to the nature of the pigments, and 

 that such names as ' diatomin ' are to a great 

 extent meaningless. Kohl ('06) has shown that 

 the pigment in the chromatophores of diatoms 

 consists of chlorophyll, carotin and xanthophyll. 

 Molisch ('05) asserts that leucocyanin also 

 occurs in the chromatophores of diatoms, and 

 that it is this pigment which causes these 

 plants (and also an alcoholic solution of 

 ' diatomin ') to turn blue-green on the ad- 

 dition of hydrochloric or sulphuric acid. 



Pyrenoids are found in the chromatophores 

 of diatoms, but they are variable both in 

 number and disposition. They consist of an 

 albuminous protein and are usually lens-shaped. 

 Not infrequently they are grouped in clusters, 

 and they often project internally from the 

 chromatophores, Mereschowsky ('03) having 

 recorded instances in which they have partially 

 or entirely emerged from the chromatophores, 

 appearing as free, colourless bodies on their 

 inner surfaces. 



In most diatom cells there are drops of a 

 fatty oil which are generally more conspicuous than the pyrenoids. This oil 



Fig. 69. A, Girdle view of Nitz- 

 schia siymoidea (Ehrenb.) W. Sm. 

 showing two large chromato- 

 phores with oil globules, and 

 also the nucleus. B and C, 

 Valve views of Cyclotella compta 

 (Ehrenb.) Kiitz. showing nume- 

 rous discoidal chromatophores. 



D, Isolated perforated chromato- 

 phore of Gyrosigma balticum 

 (Ehrenb.) (after O. Miiller). 



E, Girdle view of Synedra Ulna 

 (Nitzsch) Ehrenb. showing the 

 irregular chromatophores. ch, 

 chromatophores ; n, nucleus ; 

 ol, oil globules. 



