MICROSCOPIC FORMS OF VEGETABLE LIFE. 



277 



289), and of Mr. Shadbolt on Arachnoidiscus ( 291) leave no donbfc 

 that in those Diatoms the silicified valve is composed of two layers; and 

 the same is probably the case in all those forms which present a surface- 

 areolation. Appearances are seen, too, in other Diatoms, which seems to 

 indicate that in them also the valve consists of two layers. 1 



277. The ' beaded' aspect (Fig. 166, A), which is generally, if not 

 universally, discernible in the silicified envelopes of Diatoms, when ex- 

 amined under a sufficiently high magnifying power, and with an illumi- 

 nation specially adapted to display them, is now usually regarded as in- 

 dicating that the silicified envelope is composed of globular particles of 

 silex, closely set together in regular rows. 2 And on this view of their 

 nature, it is on the "dimensions of their component spherules, and on the 

 mode in which they are disposed, that those peculiar markings of certain 

 Diatom-valves depend, which render them of special value as Test-objects 

 ( 161). Such valves have been commonly spoken of as marked by stria, 

 longitudinal, transverse, or oblique, as the case may be; but this term 

 does not express the real nature of the markings (the apparent lines 

 being resolvable by Objectives of sufficient magnifying power and angular 

 aperture into roivs of dots), and should only be used for the sake of con- 



Triceratium favus:A., side view ; B, front view. 



cisely indicating the degree of their approximation. If we examine 

 Pleurosigma angulatum, one of the easier tests, with an Objective of 

 l-4th inch focus (having an angular aperture of 90 and a magnifying 



1 See Mr. C. Stodder (of Boston, U. S.), <; On the Structure of the Valve of the 

 Diatomacece," in "Quart. Journ. of Microsc. Science," Vol. iii., N.S. (1863), p. 

 214; also Ralfs, Op. cit., Vol. vi. (1858), p. 214; and Rylands, Op. cit., Vol. viii. 

 (1860), p. 27. 



2 See Dr. Wallich's Papers on this subject in " Quart. Journ. of Microsc. Sci- 

 ence," Vol. vi. (1858), p. 247; " Annals of Nat. Hist.," Vol. v. Ser., 4 (Feb., 1860), 

 p. 122; and "Trans, of Microsc. Soc.," Vol. viii., N.S. (1860), p. 129. See also 

 Norman in " Quart. Journ. of Microsc. Sci.," Vol. ii., N.S. (1862), p. 212. Mr. 

 Wenham, who at one time inclined to the opposite belief, stated (when Dr. Wal- 

 lich's Paper was read before the Microscopical Society), as the result of observa- 

 tions made with an Objective of l-50th inch focus and large aperture, that the 

 valves are composed wholly of spherical particles of silex, possessing high refrac- 

 tive power; and he showed how all the various optical appearances presented by 

 the different species could be reconciled with the supposition that their structure 

 is universally the same. Recourse has been had, with a certain measure of suc- 

 cess, to the production of ' artificial Diatoms ' by the deposit of silex from its fluo- 

 ride; thin films being formed, which exhibit a ' beaded' structure, often arranged 

 in very regular patterns. See the Memoir of Prof. Max Schultze, abstracted in 

 "Quart. Journ. of Microsc. Sci., N.S., Vol. iii. (1863), p. 120; and Mr. Slack's 

 Paperin "Monthly Microsc. Journ.," Vol. iv. (1870), p. 181. 



