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 toformascum. They form a large 
Fic. 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. 
