301 



Ox the Structure of the Valve of Pleurosigma. 



By T. F. Smith. 



(Read September 28th, 1888.) 



Plate XXIII. 



From the time since the development of the microscopic 

 objective rendered the study possible, two rival schools of 

 microscopists have brought their observations to bear on the 

 question of diatom structure, with the result that one school 

 soon saw nothing but beads, and the other nothing but perfora- 

 tions. On account of their coarser structure — that is of the 

 primary — the so-called markings of such diatoms as Triceratium 

 and Coscinodiscus were at an early date settled as being unmis- 

 takably perforations ; but the structure of the finer forms, such 

 as Pleurosigma, still remains with many an open question. The 

 two opposite schools of interpretation were well represented 

 here, in one night, by Messrs. Morland and Deby, who both read 

 papers on " Diatom Structure." * Indeed, I may say that a 

 third and newer school was represented, for Mr. Deby, after going 

 into detail to prove that the diatom frustule is in all cases an 

 hermetically closed envelope, proceeded to caution observers to 

 remember that, owing to the objective not taking up all the 

 diffraction spectra, the character of the image is likely to be 

 something very different from the real structure. This last 

 theory is the negation of all belief ; but in spite of it I think 

 the opinion of the majority of those who have given their minds 

 to the study, has crystallized into the form that diatoms are built 

 up, mostly, of perforated plates of silex, and may consist of one 

 or more layers ; combining thus great strength with lightness 

 and economy of materials. But it is one thing to formulate a 

 theory, and quite another to make facts agree with it ; and fix 

 it how you may, there are some outstanding appearances which 

 refuse to lend themselves to the theory of perforations, and 

 leaves the mind in a state of perplexity on the subject. 



I believe I betray no confidence if I say that Mr. Nelson him- 

 * " Journ. Q.M.C.," Ser. II, Vol. ii, pp. 297 and 308. 



