DIATOMS 



331 



movements, reproduction by longitudinal fission, and such 

 structures as contractile vacuoles and red pigment spots suggest 

 a relationship to the animal kingdom. Consequently, they are 

 regarded as a transition group between plants and animals. 



Diatoms. — These one-celled plants are often classed with the 

 Brown Algae on account of their brown pigment, although they 

 differ from the Brown Algae in a number of ways. The Diatoms 

 are a vast assemblage of plants varying widely in form and 

 occurring in vast numbers in fresh water, salt water, and on damp 

 soil. They float or swim commonly on the surface of water and 

 often in such vast numbers as to form a scum. They form a large 



Fig. 285. — Diatoms of various kinds (X 30-200). In cases where a pair 

 of individuals equal in length are shown, two views of the same Diatom are 

 included. From Kerner. 



part of the floating plankton or free-swimming organic world on 

 the surface of the ocean. Many occur as fossils and their silicified 

 walls form a large part of the deposits of siliceous earth in which 

 form they are used in the manufacture of dynamite, scouring 

 powders, etc. Some are free-swimming while others are attached 

 by stalks. 



The plant body is microscopical and may have most any shape 

 imaginable as may be seen from Figure 285. The cell wall, 

 consisting largely of silica, is very rigid and durable and is com- 

 posed of halves which fit together one over the other much like 

 the two parts of a pill box. The walls of some are delicately 

 but beautifully marked with fine cross lines, which make certain 

 Diatoms suitable objects for testing the definition of microscopes. 



