428 A TEXTBOOK OF BOTANY [Ch. X 



the most diversified and beautiful of all objects in nature 

 (Fig. 296). Their markings are partly raised ribs or tuber- 

 cles, and partly sunken pits, slits, or pores. Unlike the 

 Desmids, their cellulose walls are densely infiltrated with 

 silica, which reproduces and holds the form of the wall so 

 perfectly that these beautiful flint skeletons persist and 

 accumulate long after the living substance is decayed, or 

 after the plants are burnt. Many small lakes are filled 

 nearly to the water level with a "'mud," which is composed 

 chiefly of living Diatoms near the top, of their partially de- 

 cayed remains lower down, and of pure white empty shells 

 at the bottom; and from such ponds, and from fossil de- 

 posits in like ponds of old time, is taken the " diatomaceous 

 earth," "infusorial earth" or "fossil flour," which forms the 

 basis of most polishing powders, and has other minor uses 

 in the arts. The plant body, while typically a single cell, 

 is sometimes a colony in form of a filament, or even a sym- 

 metrical frond, the individuals being embedded in a gelati- 

 nous material they secrete. The cell contains chromato- 

 phores, from one to many, usually simple in form, and 

 brownish yellow in color on account of admixture of other 

 pigments with the chlorophyll. They reproduce asexually 

 by division, usually lengthwise of the cell. The usual di- 

 rect fission is of course prevented by the siliceous framework 

 of the wall, but a remarkable compensating feature is found 

 in the method of its construction. It consists of two 

 box-shaped parts, of which one overlaps the other just as 

 the cover slips over a paper box (Fig. 297). The enlarging 

 protoplasm pushes apart the valves, divides lengthwise, 

 and each half develops a new valve on its naked side. 

 Since, however, this new valve is necessarily formed with 

 its edges inside the older, each successive division reduces 

 the size of the cell ; but when this reduction has reached a 

 certain degree, the protoplast sheds both valves, and, either 

 with or without conjugation, becomes a special body called 

 an auxosporEj which grows to the full original size before 



