THE DIATOMS 195 



fusion of processes, one put out from each gamete (Fig. 187, A). 

 The gamete protoplast of one of the cells in the pair then 

 passes over into the other, or in certain forms the two protoplasts 

 unite more or less midway between the two cells (Fig. 187, C). 

 In some species, however, there is a conjugation between adja- 

 cent cells of the same filament (Fig. 187, B), the contents of 

 one cell entering the other. The fusion of the two gamete pro- 

 toplasts results in a zygospore, which develops a heavy wall 

 about itself as in the desmids and, as a resting spore, carries 

 the plant over unfavorable seasons. On germination each zygo- 

 spore puts forth a filament, which grows by repeated cell divisions 

 all along its length. 



ORDER 4. THE DIATOMS, OR DIATOMALES 



226. The diatoms. The diatoms (order Diatomales) comprise 

 a remarkable group of one-celled plants, containing several thou- 

 sand species, everywhere present in both fresh and salt water. 

 They compose the greater part of the floating microscopic life, 

 called the plankton, of the ocean and lakes, and are the most 

 important source of food for some of the smaller animal forms, 

 and through them for the fish life. The cell walls are filled with 

 the mineral silica and consist of two parts, called valves, which 

 fit together something like the halves of a pill box (Fig. 188, A). 

 Diatom cells have a great variety of forms, elliptical and cir- 

 cular, wedge-shaped and triangular, cylindrical and rhomboidal 

 (see Fig. 188). The cells are solitary in many forms, but in 

 others are arranged in filaments, or borne at the ends of gelati- 

 nous stalks, or held in filamentous sheaths or jelly-like masses. 

 Many of the diatoms, and especially the boat-shaped forms, 

 glide to and fro in the water. The cells contain chromato- 

 phores which are generally colored brown (although certain 

 species are green), but in spite of this color the most natural 

 position of the diatoms seems to be among the green algae, with 

 possible distant relationships to the desmids. 



