CELL-WALLS 155 



one valve (c. ?v} fits over that of the other (c. w'} like the 

 lid of a pill-box. The cell-wall is impregnated with silica, 

 so that diatoms can be boiled in strong acid or exposed to 

 the heat of a flame without losing their form : the protoplasm 

 is of course destroyed, but the flinty cell-wall remains 

 uninjured. 



Moreover, the cell-walls of diatoms are remarkable for the 

 beauty and complexity of their markings, which are in some 

 cases so delicate that even now microscopists are not agreed 

 as to the precise interpretation of the appearances shown 

 by the highest powers of the microscope. Two species are 

 shown in Fig. 36, D and E, but, in order to form some con- 

 ception of the extraordinary variety in form and ornamenta- 

 tions, specimens of the mounted cell-walls should be ex- 

 amined and the plates of some illustrated work consulted. 

 See especially Schmidt's Atlas fur Diatomaceenkunde and 

 the earlier volumes of the Quarterly Journal of Micro- 

 scopical Science. 



We see then that while Diatoms are in their essential 

 structure as simple as Haematococcus, they have the power 

 of extracting silica from the surrounding water, and of 

 forming from it structures which rival in beauty of form and 

 intricacy of pattern the best work of the metal-worker or the 

 ivory-carver. 



