OF THE DIATOMEiE. ~ 91 



This critique of Meneghini loses much of its force when it is noticed that 

 the existence of a pedicle, or isthmus, or of a muco-gelatinous sheath envelop- 

 ing the frustules, is assumed by him, quite h}^:)othetically, to indicate their 

 formation within a cell-wall represented by the soft investment, — an idea 

 originated by him because he could not admit of an extra-cellular formation. 

 The present state of knowledge, however, clearly recognizes the not infi-equent 

 formation of extra- cellular matters about cells, and consequently this portion 

 of the difficidty in question will cease to have importance. 



On the other hand, no animals can be pointed out having a similar complex 

 silicious stmetiu'e, whilst an analogy may be, to a certain extent, foimd with 

 the Desmidiea), some of which have a small deposit of silica in their envelopes, 

 which again in some Diatomaceous frustules is very deficient (see p. 37). 



Indeed, the affinity between the Dcsmidiese and the Diatomea) is manifested 

 by the diffferential characters vrhich naturalists feel themselves called upon to 

 indicate (see p. 95). 



The composite structure of the fnistules is principally the residt of the per- 

 meation of the external timic with silex. The little box or capsule, when 

 first produced, represents a simple enclosed cell, imbued with more silica than 

 a Desmidiaceous frond, but otherwise not histologically unlike. When the 

 Httle being prepares for self-di\'ision, the opposite valves separate, much as 

 the opposed halves of a frond of one of the Desmidiea?, and the intermediate 

 production, according to the habit of the class, becomes penetrated by silica 

 (to a less extent, however, than the original valves), and assumes so much of 

 a permanent character that it is very frequently considered an independent 

 tliii'd segment. 



So again, the cellular, or areolate, or otherwise figured and involuted sui-face 

 of the frustules, cited by Meneghini as dissimilar to any plant- structure, would 

 also ajjpear to be a consequence of this permeation of the organic membrane 

 with silica, and of various modifications consequent thereon. To show that 

 analogies are not wanting in the vegetable kingdom of ciuiously modified and 

 figured cell-waUs, we may mention as examples, besides poUen-grains, in- 

 stanced by Kiitzing, the sporangia of Desmidiea? and of various Algae. More- 

 over, the capability of the simplest enclosing membrane to develope a very 

 complex superficial structm^e is illustrated in the case of the Rhizopodes, 

 among which are many examples of striated, areolated, and other-wise modified 

 shells, which, in the eyes of many, range "with imiceUular organisms. We 

 must not forget to state that Meneghini himself seems to have appreciated 

 the eff'ect of the permeation of silica upon the characters of the ceU-wall ; for 

 he says, in liis supplementary annotations {op. cit. p. 511), " the part which 

 sHex takes in the formation of the cell- wall is undeniable," as in the epi- 

 dermis of Gramineae, Palms, and Equiseta. " The stomatic cells of Equiseta 

 merit particular attention, both fi'om the silex they contain, and the transverse 

 striae they present on the internal surface. This resemblance to the shield af 

 Diatomeae might lead us to believe that we ought to regard it as an argument 

 for maintaining the vegetability of the latter: but I do not think that I ought 

 to dwell upon such an objection ; I only notice it because I would not appear 

 to be, or pretend to be, unacquainted with it. Yet it seems to mo important in 

 another point of view — the apparent complication that the simple cell may 

 assume when penetrated by silica." 



We cannot do better than close this part of the argument by Prof. Smith's 

 review of the subject (Si/noj^s. ii. p. xix) : — '' In every case this membrane 

 [of the frustule] is more or less penetrated or imbued with silex; and the 

 presence of this substance appears to have modified the intimate structiu^e of 

 the membrane, and induced great variety in the mode and character of its 



