240 Transactions of the [Sess. 



Instead of possessing a simple or compound pedicel or sti23es, 

 many Diatoms — which Ehrenberg included under the name " Lacer- 

 nata " — are embedded in a gelatinous substance ; and since numer- 

 ous frustules are often embraced in this covering, frondose appear- 

 ances are the result. These frondose forms may be either mem- 

 branous, filiform, or filamentous and ramulose, — differences in 

 thickness and in expansion being at the same time often clearly 

 recognisable. Eeinhardt ^ remarks that the modes of formation of 

 the pedicel of stipitate species and of gelatinous colonies present 

 phenomena analogous to those found in palmellaceous Algse, inter- 

 calation of cell-walls being sometimes found in Mastogloia, in 

 which outer gelatinous and inner more consistent layers are to be 

 recognised, a pedicel being the result of local mucilaginisation of 

 the outer layer. AVithin the muco-gelatinous envelope the dis- 

 position of the frustules may be either regular, as in Schizonema 

 — a not uncommon marine genus — or irregular, as in the marine 

 Dickieia. In the determination of species, the flaccid or rigid con- 

 sistence of the fronds, the character of the branching, and the 

 nature of the union of the ramuli into fascicles, are important. 



The siliceous substance which is incorporated in the cell-walls 

 of living Diatoms varies very greatly in its amount in different 

 genera. In some cases, such as Ebizosolenia and Thalassiosira, 

 it is very deficient, the wall being even in a (/uasr flaccid condition ; 

 and, generally speaking, there is less siliceous matter in filamentous 

 than in free forms, and less in tubular processes than in the walls 

 of the frustule bearing these processes. The silex is associated 

 with organic matter, and, according to Frank! and, with iron as 

 silicate or protoxide — a circumstance which explains the occurrence 

 of a brown colour when Diatoms are moderately heated, the prot- 

 oxide absorbing oxygen and becoming brown peroxide. 



In addition to the siliceous lamina, various other membranes 

 or cell-tunics have from time to time been recorded, and among 

 these it is interesting to recall (1) the soft primordial membrane 

 enclosing the cell contents ; (2) Nageli's third tunic, or mucila- 

 ginous layer, inside the primordial layer ; and (3) Kiitzing's third 

 or cement tunic, extending often over a considerable part of the 

 frustule outside the siliceous coat. This layer is to be regarded, 

 according to Kiltzing, as equivalent to the connecting matter of 

 the frustules when these occur in union ; while Frankland believes 

 that the stains on whose existence it was established are attribut- 

 able to the oxidation of iron salts in chemical union with silica. 

 On the other hand (4) Shadbolt's third tunic, which has been 

 said to be sometimes " cellular," and to present markings different 

 from those of the siliceous valves below it, was regarded as horny, 

 somewhat pliant, and possessing a certain amount of elasticity, so 



1 ' Sitzuiigb. Versamml. Russ. Naturf. u. Aertze.' Odessa: 1S83. 



