Colony formation 



97 



D E 



Fig. 70. D and E, girdle and valve 

 view of Navicula viridis Kiitz. 

 showing chromatophores (ch) and 

 nucleus (n) ; F, Eunotia gracilis 

 (Ehrenb.) Eabenh. girdle view show- 

 ing chromatophores and nucleus. 

 ( x 400.) 



is easily soluble in ether and is blackened in osmic acid. It can be shown 



to be a food-reserve by keeping diatoms for 



a considerable period in tap-water in closed 



vessels, in which case it is completely used 



up. There are also so-called ' oil-drops ' of 



another nature. These are often of larger 



size, and they do not dissolve in ether, 



neither do they become black with osmic 



acid. Their function is obscure and they 



are known as Biltschli's red corpuscles. 



The normal nutrition of diatoms is holo- 

 phytic, but several saprophytic forms have 

 been described. In the latter there is a 

 complete absence of pigment, and such forms 

 occur for the most part where there is an 

 abundance of decaying organic matter in the 

 water. Benecke ('00), who obtained the 

 colourless diatoms Nitzschia putrida, and 



N. Leucosigma from Kiel Harbour, affirms that there are no transitional 

 states between brown and colourless diatoms. Most of the known sapro- 

 phytic forms are extremely motile. Karsten ('01) succeeded in producing 

 a saprophytic form of Nitzschia palea by cultivating it in favourable nutritive 

 media such as glycerin or grape-sugar. Nitzschia putrida has become 

 so far saprophytic that all attempts to induce the formation of chromato- 

 phores in this diatom have failed. Synedra hyalina Provazek ('00) is another 

 colourless diatom. 



There is evidence to show that some diatoms do not require calcium, but 

 that sodium is essential. 



THE BUILDING OF COLONIES AND THE SECRETION OF MUCUS. Diatoms 

 are frequently solitary and free-floating, but they are also united in 

 various ways to form colonies, generally of small size. In some of these 

 colonies the individuals are united by their valve-faces, in which case they 

 form ribbon-like (Eunotia, Tetract/clus, Fragilaria, Rhabdonema) or thread- 

 like (Melosira, Bacteriastrum, Ch&toceras) chains. In others the cells are 

 joined in an irregularly zigzag manner at the corners of the valves (Tabel- 

 laria, some species of Diatoma, Triceratium, arid Biddulphia). In all 

 these cases the attachment is by mucus secreted by the cells themselves. 

 The cells of the ribbon-shaped and thread-like colonies are joined either 

 by a layer of mucus or by mucous strands between the closely applied valve- 

 faces, and the cells of the zigzag colonies are united by small mucous 

 cushions at the corners of the valves. 



\V. A. 7 



