PLANT LIFE 148 



(1) Diatoms. — These organisms are found 

 throughout the world in all fresh and salt 

 water and in damp places. They occur not 

 only floating in the ocean but attached to 

 other algae and to animals in almost all regions. 

 They are distinguished from other algae in 

 possessing thin silicated cell- walls (often 

 beautifully sculptured), which consist of two 

 similar valves, or frustules, fitting into one 

 another like the top and bottom of a pill-box. 

 New cells are produced by division. Some 

 species are capable of motion, and glide over 

 the sand or mud or through the water ; in 

 other species the separate frustules glide over 

 each other with a back and forward motion. 



The pelagic species (see Plate VII.), to 

 which reference is here specially limited, 

 have generally thinner walls than fixtd shore 

 or neritic species, and have frequently highly 

 developed suspension-organs. They have 

 been divided into four groups : — 



(a) The bladder type, of which Coscino- 

 discus rex (over a millimetre in diameter) is 

 the largest. 



(b) The ribbon type, with flattened cells, 

 several cells being united together into ribbon- 

 like colonies, as in Fragillaria oceanica, 



(c) The hair type with cells very much 

 prolonged in one direction or united into 

 elongated colonies. The elongated form tends 



