276 



BOTANY: PRINCIPLES AND PROBLEMS 



from ordinary cell division, reproduction is effected by conjuga- 

 tion between two protoplasts, each of which has escaped from its 

 wall. 



The Diatoms (Fig. 155), a large group of unicellular plants 

 of somewhat uncertain relationship, can perhaps best be con- 

 sidered here. They are represented by thousands of species in 

 salt and fresh water and are the most abundant of the minute 

 plants which make up that multitude of freely swimming or float- 



FiG. 155. — Diatoms of various types. 



ing algae in the open sea and in fresh water which we know as the 

 plankton. Many species are also sessile and may be united into 

 filaments or other colonies. The wall is hard and flinty, being 

 heavily filled with silica, and consists of two halves or valves one 

 of which fits over the other like a box cover. These are highly 

 diversified in shape and are frequently ornamented with minute 

 and intricate markings which have long been the delight of micro- 

 scopists. The color of most diatoms is brown owing to the pres- 

 ence of a pigment, diatomin, in addition to chlorophyll. 



