SINGLE-CELLED PLANTS AND COLONIES. 13 



toms are very various in form, and present two different 

 aspects. When seen from the side they are generally elon- 

 gated-rectangular. When looked at from above they are 

 short-cylindric, disk-shaped, boat-shaped, or variously curved 

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

 filled with silica that scarcely any other material is left. In- 

 deed 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. iz). Each 



Fig. 12. — A single diatom (^Namcwla o-w^hirhyTichus). //, top view; B^ 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. 



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. 11). Seen in mass, as they may 

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

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

 times single and always few, have a brownish color in addi- 

 tion to the green chlorophyll. 



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- 



