SECT. II. 



D1ATOMACEA. 



197 



seen in the protoplasm. The golden-brown matter is 

 supposed to be chlorophyll, whose green tint has been 

 changed by the presence of iron, which is assimilated in 

 this group. Such is the internal structure of a race of 

 plants altogether invisible lo the naked eye. Their ex- 

 ternal forms, reproduction and movements, are no less 

 wonderful. 



The silicious envelope of the simple cell of a Diatom 

 or frustule, as a single plant is usually called, consists of 

 two valves or plates, commonly of the most perfect sym- 

 metry, closely ap- 

 plied to each other 

 along aline of junc- 

 tion like the two 

 valves of a bivalve 

 shell, and each valve 

 being more or less 

 concavo -convex, a 

 cavity is left be- 

 tween the two which 

 is occupied by the 

 golden - brown cell 

 described above. The 

 form of the cavity 

 differs greatly, for 

 sometimes each valve 



is hemispherical, SO Fig.ll. A,Diatomavulgare: a, side riew of frustule; 



. , .-, . , . b, frustule undergoing^self -division. 



tliat tne Cavity IS B. Grammatophora serpentina : a, front and side 



i i i . . view of single frustule ; 6, front and end view of di- 



glODUlar ; SOmetimeS vided fru&tule ; c, frnstule about to undergo divi- 



it is a small aton; d ' fnu * uto completely divided - 



ment of a sphere, resembling a watch-glass, so that 

 the cavity is lenticular ; in short, the form of the cavity 

 depends upon that of the valves, which may be heart- 

 shaped, or much elongated, square, triangular, boat- 

 shaped, or furnished with outgrowths, which, however, 

 is rare. 



