PROTOPHYTA 63 



tically imperishable. In consequence there is a constant settling 

 on lake or sea floor of shells coming from dead diatoms, from 

 valves discarded at reproduction and spore formation, and from 

 shells remaining after digestion and defecation by marine ani- 

 mals. Masses of such accumulations* raised above sea-level by 

 upheavals from time to time upon solidification have formed 

 masses of "diatomaceous" earth in which the shapes and mark- 

 ings of the individual diatoms are still intact. These earths some- 

 times cover an area of many miles, and at some places, as near 

 Richmond, Virginia, the beds are forty feet in depth, while in 

 some of the western States large deposits up to 300 feet in thick- 

 ness have been discovered. These earths are usually gray or 

 white in color, and being quite easily pulverized they have sev- 

 eral economic uses, as for example in so-called tripoli polishing 

 powders (Kieselguhr) , dentifrices, non-conducting materials, or 

 as absorbents for nitro-glycerine in the manufacture of dynamite. 



Each diatom may be observed from two aspects : 1, from that 

 in which one or the other valve is seen (valve view) ; 2, from that 

 in which the girdle is exposed to view (girdle view). In the 

 majority of diatoms (the Pennatae) the face of each valve is 

 marked by a central line (raphe) which is interrupted in the mid- 

 dle by a nodule, and terminates in a nodule at each end. Delicate 

 markings, composed of minute pores in the shell, are arranged 

 symmetrically on the two sides of each raphe and find useful ap- 

 plication as test objects for microscope lenses. This type of 

 diatom has the power of independent movement which, according 

 to the observations of competent authorities, is due to the secre- 

 tion of a gelatinous material which comes out through these deli- 

 cate pores and through the raphe. In another and a smaller 

 group (the Centricae) , most of which are marine and motionless, 

 the valves are circular and without central lines, the valve mark- 

 ings being arranged radially with respect to a central point. 



The chromatophorest vary from one to many in number and 

 are also highly variable in form. Among the products of assimi- 

 lation stored up in the cell are variously colored droplets of oil. 

 These are probably the source of occasional unpleasant odors 

 and tastes in drinking waters. The genus Asterionella, for ex- 



* See "The Plant World," pages 16-17, in this Series. 

 t See the Glossary of this book, page 120. 



