94 



BacillarieaB 



length and placing it in communication with the exterior, while it is 

 connected with the interior of the cell by cross-canals (fig. 66). The free 

 edge of each wing is thus somewhat thickened to accommodate the raphe- 

 canal, and its flat portion consists of alternating cross-canals (fig. 66 qk) and 

 thin intercalary pieces (fig. 66 zw). 



In Nitzscliia there is a raphe-canal with a longitudinal fissure similar 

 to that of Surirella, extending along the whole length of the keel of each 

 valve. 



In some diatoms there is a narrow, hyaline, axial area without a central 



nodule, either median or submedian in position. 

 This is not a true raphe, since there is no cleft 

 in the valve, and it is known as a pseudoraphe. It 

 may be present on both valves, or, as in Cocconeis, 

 Achnanthes (fig. 67), and Rhoicosphenia, limited to 

 one valve only, the other valve possessing a true 

 raphe. In Dictyoneis, Vanheurckia, and some of the 

 larger forms of Stauroneis, the raphe is enclosed 

 between a pair of siliceous ribs. 



THE PROTOPLAST. There is a well-marked lining 

 layer of colourless cytoplasm, which extends into the 

 various extensions of the cell-cavity, and also into 

 the various chambers, pores, and canals in the cell- 

 wall. In most diatoms there is one large vacuole 

 occupying the greater part of the interior, and extending right through it in 

 the median part of the cell is a cytoplasmic bridge in which the nucleus 

 is usually embedded. The general form and position of this bridge varies 

 much in different diatoms, but is fairly constant for the same species. 



The nucleus is narrowly ellipsoidal, subreniform, or sometimes almost 

 fusiform, in the pennate diatoms, but it may be quite spherical in the centric 

 species. It contains one or more fairly conspicuous nucleoli, and Lauterborn 

 ('96) has demonstrated the presence of a centrosome in certain of the larger 

 species of Navicula and Surirella. Other authors have described the 

 presence of a macro- and a micronucleus in certain diatoms. Lauterborn 

 attempts to reconcile the various interpretations of nuclear structures said to 

 be observed in diatoms, and states that it is highly probable that at one time 

 the cell possessed two equally constructed nuclear bodies, but that gradually 

 a division of labour has brought about a greater and a greater differentiation 

 until one has acquired a purely morphological and the other a purely 

 physiological function. The macro- and micronucleus of certain authors are 

 possibly equivalent to the nucleus and centrosome of Lauterborn. This 

 author has also followed out the mitotic division of the nucleus of several of 

 the larger diatoms, in which there is apparently a relatively large number 



Fig. 67. Achnanthes bre- 

 vipes Ag. var. intermedia 

 (Kiitz.) Cleve. Twovalves 

 of one cell ; A with raphe, 

 B with pseudora,phe. 



