328 N. E. BROWN ON THE STRUCTURE OF DIATOMS. 



in consequence of having seen the minute pores in P. balticum, I 

 eagerly examined, as I was reminded that some four years ago 

 whilst examining S. gemma mounted in styrax with a Leitz 

 -jL.th achromatic oil-immersion objective of 1*3 N.A. I had seen 

 similar pores or dots on the white beads of that diatom. At the 

 time, being very busy with other work and thoroughly accepting 

 the opinion that the black dots usually seen were pores, I paid no 

 attention to what I then saw. Now, however, I examined them 

 with fresh interest and found that in this realgar mount the pores 

 are distinctly visible. To see them, the valve must be resolved 

 into a grating formed of slender, slightly zigzag black bars, with 

 the interspaces divided by very slender transverse partitions into 

 small meshes (the so-called white-dot focus). At a magnification 

 of from 1,800 to 3,000 diameters on some specimens, but not all, a 

 minute dark speck or pore at the centre of every one of the 

 meshes is very clearly visible (figs. 3 and 4) ; at the same time 

 it is so minute that it requires good eyesight to perceive it, 

 but, as in other cases, becomes accentuated if a small stop be 

 placed in the carrier of the condenser. There is therefore no very 

 great difference in the ultimate structure of this Surirella and of 

 Pleurosigma balticum, except that in the latter it is the bars 

 parallel to the longer axis of the diatom which are most evident, 

 whilst in Surirella gemma the bars transverse to that axis are the 

 most apparent. It must be understood that I refer here only to 

 the fine secondary bars or those of the cell-wall, not to the stout 

 primary bars which form the framework of the diatom and sup- 

 port the cell- wall. The nodes, formed by the junction of the 

 slender partitions with the bars, at another focus produce the ap- 

 pearance of black dots by refraction or diffraction as they do in 

 Pleurosigma balticum. One specimen of S. gemma on the slide is 

 crumpled up and the bars bent and turned aside so as to show 

 their nature very clearly when sufficiently magnified, and demon- 

 strate that they are exactly of the same character as those of 

 Pleurosigma balticum that is, they are the strengthening bars of 

 the membrane of the diatom. I have been unable to determine 

 whether there is also a membrane over the inner surface of these 

 bars, but think it very probable, in which case the white bead- 

 like appearance will be chambers with minute orifices in their 

 iuner and outer wall. 



This diatom seems to provide the microscopist with a series of 



