PRINCIPAL GROUPS OF PLANTS. 35 



culture medium in bacteriology and in other work where a nutrient 

 is desired. It occurs commercially in bundles 4 to 6 decimetres in 

 length, consisting of thin, translucent, membraneous, agglutinated 

 pieces, yellowish-white in color. It is usually brittle, but becomes 

 tough when moistened. It is used in medicine in the powdered 



FIG. 18. Arachnoidiscus Ehrenbergii, a characteristic Diatom found in Agar-agar. From 

 a photomicrographic negative by J. J. Woodward, Surgeon, U. S. A. 



form. Under the microscope Agar-agar frequently shows the 

 frustules or siliceous cell walls of diatoms, which are disk-shaped 

 (Fig. 18). It is insoluble in cold water, but dissolves slowly in 

 hot water. Upon boiling I part in 100 parts of water it should 

 yield a stiff jelly upon cooling. 



Diatoms constitute a large group of unicellular plants, occur- 



