2] 



THE VARIOUS (GROUPS OF PLANTS 



33 



Bacillariophyceae : These are the Diatoms, famiUar to all 

 microscopists, and comprise a large and important class of Algae that 

 are all unicellular and microscopic, occurring singly or attached in 

 filaments or chains, or grouped into colonies. The form is extremely 

 various, as may be seen from Fig. 6, 

 which shows a range of different types. 

 The cell-wall is composed of two 

 * valves ', overlapping one another like 

 the halves of a pill-box, and is impreg- 

 nated with silica. Its surface is finely 

 and often beautifully sculptured, with 

 extraordinarv regularity and precision. 

 The chloroplasts contain a brown pig- 

 ment in addition to the all-important 

 chlorophyll, and accordingly the colour 

 of Diatoms both individually and en 

 masse is usually a shade of brown or 

 olive-green. 



The nutrition of Diatoms is primarily 



A B 



Fig. 6. — Diatoms (Bacillariophyceae). A, various forms (variously magnified); 

 B, a colonial type, Licmophora flabellata ( •: 70). 



by photosynthesis, food being stored in the form of oil. Their 

 reproduction is normally by cell division, though occasionally sexual 

 conjugation takes place, followed by the production of special 

 ' auxospores ', or these latter may be produced apomictically ; 

 alternatively, small flagellated gametes or spores may^be formed. 

 Diatoms are abundant in both marine and fresh waters in practically 

 all climates, forming a significant, and often the main, constituent 



