274 GENERAL CHARACTERS OF DIATCOJACE.E. 



of the siliceous envelope has been found by Prof. Bailey to remain 

 after the removal of the silex by hydrofluoric acid ; and although 

 this membrane seems to have been presumed by him, as also by 

 Prof. W. Smith, to lie beneath the siliceous envelope, and to secrete 

 this on its surface as a sort of epidermis, yet the Author agrees 

 with the authors of the " Micrographic Dictionary" (p. 200), in 

 considering it much more likely that it is the proper Cellulose wall 

 interpenetrated by silex ; especially since it has been found by 

 Schmidt, that after removing the protoplasm of Frustulia salina 

 by potash, and the oil by ether, a substance remains identical in 

 composition with the Cellulose of Lichens. Moreover, there are 

 several Diatoms in which, as in A racJinoi discus (§ 229), a pellicle 

 of vegetable membrane of horny consistence, having markings of 

 its own quite independent of those of the silicified layer, overlies 

 the latter ; and it is probably never entirely absent, although it is 

 sometimes thin enough to be removed by a few seconds' immersion 

 in boiling nitric acid. Hence, as Prof. Walker Arnott has justly 

 observed,* the appearances presented by individuals of the same 

 species vary greatly, according to the treatment to which they have 

 been respectively subjected ; and no certainty can be obtained in 

 the discrimination of Species, except by the comparison of recent 

 specimens, 1st, after being immersed for a short time in cold nitric 

 acid, or simply washed in boiling water ; 2nd, after being boiled 

 in acid for about half a minute, or a whole minute at most ; 3rd, 

 after being boiled for a considerable time. Thus it is obvious that 

 specimens obtained from Guano or from Fossilized deposits can 

 only be rightly compared with Kecent specimens, when the latter 

 have been subjected to a treatment whereby their organic matter 

 shall be removed as completely as possible. 



212. The Endochrome of Diatoinaceoe, instead of being bright 

 green, is of a yellowish brown ; and its peculiar colour seems to be 

 in some degree dependent upon the presence of iron, which is 

 assimilated by the plants of this group, and may be detected even 

 in their colourless silicified envelopes. The Colouring substance 

 appears to be a modification of ordinary chlorophyll ; it takes a 

 green or greenish-blue tint with sulphuric acid ; and often assumes 

 this hue in drying. The endochrome consists, as in other plants, 

 of a viscid Protoplasm, in which float the granules of colouring 

 matter. In the ordinary condition of the cell these granules are 

 diffused through it with tolerable uniformity, except in the central 

 spot, which is occupied by a Nucleus ; round this nucleus they 

 commonly form a ring, from which radiating lines of granules 

 may be seen to diverge into the cell-cavity. At certain times, Oil- 

 globules are observable in the protoplasm ; these seem to repre- 

 sent the starch-granules of the Desmidiacea? (§ 200) and the oil- 

 globules of other Protophytes (§ 182). A distinct movement of 

 the granular particles of the endochrome, closely resembling the 



" Quarterly Journal of Microscopical Science," Vol. vi. (1858), p. 163. 



