S1XGLE-CELLED PLANTS AND COLONIES. 15 



or angular. They are peculiar in having the cell-wall so im- 

 pregnated with silica that scarcely any organic matter is left. 

 Indeed the plants may be heated to a red heat and boiled in 

 acid without destroying the form and markings of the cell- 

 wall, so completely has it become silicified. To permit 

 growth this rigid cell-wall is constructed in two pieces which 

 fit together like the two parts of a pill-box (fig. 21). Each 

 of these pieces, or valves, is sculptured into regular patterns 

 in lines and dots, which are often so excessively minute or 

 close together as to be barely visible with the highest powers 

 of the microscope (b> fig. 20). Seen in mass, as they may 

 often be on the sides of a glass aquarium, living diatoms 

 appear yellowish-brown. The chloroplasts, which are some- 

 times single and always few, contain a brownish pigment 

 (dia/omin) in addition to the green chlorophyll. 



Fig. 2i. — A single diatom (Navicula amphirhynchus). A } top view ; />, side view, 

 showing overlapping of the valves. The parts shaded by lines are the chloroplasts; 

 the dotted part the protoplasm, with nucleus about the center of cell. Magnified 

 750 diam. — After Pfitzer. 



It is not uncommon for the diatoms to form colonies by 

 the adhesion of several or many individuals by means of 

 gelatinous cell-walls. These colonies are ribbon-like, or zig- 

 zag chains, or even branched filaments (//, i, fig. 20). 

 Other sorts may be attached singly or in clusters by a gelati- 

 nous stalk (e, fig. 20). In all cases the jelly, like the rest 

 of the cell-wall, is a product of the protoplasm. The slow 



