Structure of cell-wall 



91 



chambers. The inner walls of the secondary chambers possess a number 

 of poroids, and pore-canals are present here and there, passing rather 

 obliquely through the separating ridges (fig. 62 B and G). 



In some diatoms, such as Stephanopyocis Palmeriana (fig. 63), Sceletonema 

 costatum, and Lauderia annulata, 0. Miiller ('01) 

 has shown that the spines by which the cells are 

 united to form chains are really tubes, through 

 which there is a protoplasmic continuity from cell 

 to cell. 



The most numerous and best-developed pores 

 are found in the centric diatoms. 0. Miiller has 

 suggested that in certain species they are in 

 relation to an extracellular layer of cytoplasm 

 which brings about a centrifugal thickening of 

 the valve, but it is by no means certain that they 

 always serve for the passage of protoplasm. 

 Karsten has arrived at the conclusion that the 

 extracellular layer of cytoplasm has a morphogenic 

 activity only in certain of the pelagic genera, such 

 as Coscinodiscus, Planktoniella, Valdiviella, etc. 

 Perhaps the comparative absence of pores from 

 the pennate diatoms is to be associated with the 



presence of the raphe, which in some of these Fi g- 64. A, Valve view of Stau- 

 f , i'in i roneis acuta W. Sin. showing 



forms attains a high development. stauros. B and c, valve and 



For the careful study of the finer structure 

 of the cell -wall O. Miiller recommends treat- 

 ment with hot sodium carbonate and potassium 

 hydrate. 



The valves of many pennate diatoms, especially 

 those of the Naviculacese, possess small internal 

 thickenings which are of a somewhat rounded or 



conical shape, generally contain a cavity, and are known as nodules. They 

 occur at each extremity of the valve and in the centre, and in many of 

 the pennate diatoms are connected by a median line known as the raphe. 

 The central nodule is sometimes expanded laterally to form a stauros, which 

 may be simple (Stauroneis ; fig. 64 A) or forked (Schizostauron ; fig. 64 B 

 and C); in other cases it is prolonged into paired horns (as in Diploneis). 

 In Amphipleura it is greatly extended in a longitudinal direction. Frequently 

 there are smooth areas on a valve which is otherwise striated. Such hyaline 

 areas are generally round the central nodule and on each side of the raphe. 

 The former, which is known as the central area, often extends across the 

 median portion of the valve from one margin to the other; the latter is 



girdle views respectively of 

 Schizostauron Crucicula Grun. 

 showing the forked stauros. 

 All x 500. v, valve; iv., 

 intercalary valve with short 

 septa (s) ; cb., connecting 

 band. The fine striations on 

 the valves are not shown in 

 the figures. 



