280 



MICROSCOPIC FORMS OF VEGETABLE LIFE. 



the "hoop." The impermeability of the siliceous envelope ren- 

 ders necessary some special aperture, through which the sur- 

 rounding water may communicate with the contents of the cell. 

 Such apertures are found along the whole line of suture in disk- 

 like frustules; but when the Diatom is of an elongated form, 

 they are found at the extremities of the frustules only. They 

 do not appear to be absolute perforations in the envelope, but 

 are merely points at which its siliceous impregnation is wanting ; 

 and these are usually indicated by slight depressions of its sur- 

 face. In some Diatoms, as Surirella (Fig. 88) and Campylodiscus 

 (Fig. 87), these interruptions are connected with what seem to 

 be minute canals hollowed out between the siliceous envelope 

 and the membrane investing the endochrome. In many genera, 

 the surface of each valve is distinguished by the presence of a 

 longitudinal band, on which the usual markings are deficient ; 

 and this is widened into small expansions at the extremities, and 

 sometimes at the centre also, as we see in Pleurosigma (Fig. 80) 

 and G-omphonema (Fig. 89). This band seems to be merely a 

 portion in which the siliceous envelope is thicker than it is else- 

 where, forming a sort of rib that seems designed to give firmness 

 to the valve ; and its expansions are solid nodules of the same 

 substance. 1 



175. The nature of the delicate and regular markings, with 

 which probably every Diatom aceous valve is beset, has been of 

 late years a subject of much discussion among Microscopists, and 

 cannot be said to be even yet settled, although (in the Author's 

 opinion) the weight of evidence now decidedly preponderates on 



one side. In the first place 

 FIG. 78. it may be remarked, that 



there is a much greater uni- 

 formity in the general cha- 

 racter of these markings, 

 than was supposed when at- 

 tention was first directed to 

 them ; for what were at first 

 supposed to be lines, are now 

 resolved by objectives of 

 large angular aperture into 

 rows of dots; and these dots, 

 when suificiently magnified, 

 are found to bear a close 

 resemblance to the coarser 

 It is to the latter, therefore. 



Portion of Cell of Isthmia nervosa, highly magnified. 



markings on the larger species. 



1 These nodules were mistaken by Prof. Ehrenberg for apertures ; and in this error 

 he has been followed by Kiitzing. There cannot any longer, however, be a doubt as 

 to their real nature. As Prof. W. Smith has justly remarked : " The internal contents 

 of the frustule never escape at these points when the frustule is subjected to pressure, 

 but invariably at the suture or at the extremities, where the foramina already described 

 exist. Nor does the valve, when fractured, show any disposition to break at the ex- 

 pansions of the central line, as would necessarily be the case were such points perfo- 

 rations and not nodules.'' 



