DIATOMS 117 



which the only conspicuous portion in the fresh state 

 are a number of bodies of a brown colour. These 

 brownish bodies, though very variable in size, shape, 

 and number, according to the form examined, are 

 always present. They are called " chromatophores " 

 (colour bearers), and play a very important part in 

 the life of the plant, for they enable it to intercept the 

 light of the sun, and the " power " thus obtained is 

 used in food manufacture. 



The form of the plant itself is exceedingly variable. 

 It may be circular and disc-like, or elongated ; in the 

 latter case it may be boat-shaped, or very narrow like 

 a needle, and it may be pointed at one or both ends. 

 Some forms exhibit large projections, often of very 

 considerable length and very fine and hair-like. In 

 many forms the surface of the coat is not perfectly 

 smooth, but shows markings in the form of pits or 

 slight projections ; such markings are often arranged 

 in most regular and beautiful patterns. The advan- 

 tage obtained by the possession of these outgrowths 

 is the increase of the surface of the organism, with 

 the result that it sinks more slowly in sea water, which 

 is of less specific gravity than itself. These plants, 

 possessing no organs of movement, are at the mercy 

 of currents and of changes in the specific gravity 

 of the water. If they sink below a certain depth 

 the light which reaches them is so feeble that 

 their life-processes cannot be carried on, and they 

 soon die. Possibly, also, the spinous projections may 

 deter certain animals from devouring the diatoms so 

 provided (Figs. 33-37)- 



We find that while the majority of forms live singly, 

 others are united together into colonies, very commonly 

 forming chains (as in Cha^toceras), or having other less 

 regular forms. 



