262 Bacillariece 



so extremely fine and constant in some species as to furnish 

 splendid test-objects for the definition and angular aperture of the 

 lenses of microscopes. The valves of some genera, particularly the 

 marine ones, exhibit a beautiful areolated structure, due to the 

 presence of chambers in the siliceous cell-wall. These chambers 

 may be open to the exterior or covered by a thin membrane, and 

 their inner walls are perforated by exceedingly minute apertures 

 or pores which lead into the cell. . These pores are not present in 

 all species of Diatoms, Miiller and Lauterborn having shown that 

 in some species of Navicula (Pinnularia) they are probably absent. 

 Schiitt affirms that there is no question of the existence of pores 

 in many species of Diatoms, and their existence in a large number 

 of others is extremely probable. He draws a distinction between 

 pores and dots, and this has been further emphasized by Miiller. 

 The latter 1 has termed small circular dots which resemble pores 

 'poroids'; he gives O'l//. as the minimum diameter of pores, and 

 0'4 0*5 fj, as their maximum diameter, and all structures over 

 0'6/i he regards as ' poroids 2 .' Miiller 3 recommends treatment 

 with hot sodium carbonate and potassium hydrate in studying the 

 structure of the cell-wall. 



Heribaud 4 states that increased altitude and enfeebled light 

 cause a diminution of the number of striae and of their strength, 

 accompanied by an increase in the length and breadth of the valves. 



Many Diatoms exhibit a thickening of the cell-wall, visible in 

 the valve-view, in the centre of the valve and very often at both 

 extremities. These thickenings are known as nodules. The 

 nodules are very frequently connected by a long median line 

 known as the raphe. If the central nodule spreads out in a 

 lateral direction it is known as a stauros. A portion of the valve 

 on each side of the raphe and round the central nodule is often 

 quite devoid of striae ; this plain or smooth portion of the valve is 

 sometimes spoken of as a hyaline area. 



The raphe, for at least some part of its length, is a true cleft 

 in the valve through which the protoplasmic contents of the cell 

 are placed in communication with the surrounding medium. 

 (Consult Fig. 124r.) 



1 0. Miiller in Berichte Deutsch. Bot. Gesellsch. xvii, 1893. 



2 O. Miiller, torn. cit. xviii, 1900. 



3 0. Miiller, torn. cit. xix, 1901. 



4 Heribaud in Comptes Bendus, cxviii, 1894. 



