envelope of mucus or gelatine. But unfortunately the writers wlio
adopted this division were neither agreed as to the names to be given
to these genera, nor the species to be included under each. All the
species were at first placed by Agardh in the single genns Schizonema
(‘ Systema Algarum/ p. xv, 1824). By Dr. Greville, in the
‘ Scottish Cryptogamie Flora,’ 1826 et seq., those with secondary
tubes were retained in Schizonema, and the others placed apart in
the genus Monema. In his ‘ Conspectus Criticus Diatomacearum,’
1830, Agardh recognizes the distinction suggested by Greville, and
places the species with uncompounded fronds in Schizonema, and
those with secondary tubes in Micromega ; this arrangement, though
■ unjust to himself, was politely adopted by Greville in the ‘ British
Flora.’ Harvey, in his ‘Manual of British Algæ,’ reunites the
species under the single genus Schizonema-, and Kützing, in his
‘ Bacillarien,’ 1844, and ‘ Species Algarum,’ 1849, again divides
them according to the arrangement in Agardh’s ‘ Conspectus.’
None of these methods satisfies Ehrenberg, and he invents a new
name, Naunema, ‘ Infusionsthierchen,’ 1838, and again unites all the
species under this designation. Here are sufficient elements of confusion
for future observers. This great diversity of opinion doubtless
owes its origin to the variableness and inconstancy of the characters
adopted by the writers, who arranged the species under two genera.
I he presence of only one, or of many files of frustules, is certainly to
some extent dependent upon the stage of growth of the specimen
examined ; and the appearance of secondary tubes within the general
mucus-envelope is more or less apparent in difierent portions of the
same frond, or according as it is examined in the fresh or dry state.
A very extensive comparison of specimens leads me to believe, that,
in every case where the development of the frond is much advanced,
as in the older or basal portions, numerous files of frustules may be
discovered; and as each file, in the progress of self-division, continues
to deposit fresh mucus around its frustules, the portion of the envelope
in their immediate neighbourhood will be gradually consolidated
and present the semblance of a tube, which will become more apparent
on desiccation, as the consolidated mucus shrinks less in
drying than the neighbouring portions of the frond.
Hence has arisen in some fronds rather than others an appearance
of secondary tubes, and their.consequent generic separation by observers,
who, principally conversant with dried specimens, have regarded
this appearance of importance.
For the reasons I have ju st detailed, I am disposed to demur to this
course, and to unite all the species under one genus, the name of
which must of coursç be that first proposed by Agardh. I ought,
however, to explain why I have not accepted the characters given by
Kützing as distinctive of his two genera, viz. that the “ spermatia ”
or reproductive bodies are external in Schizonema, and internal and
immersed in Micromega. Could these characters be established, a
distinction between the two genera must at once be admitted ; but I
believe no other observer has been able to substantiate the discoveries
of Professor Kützing, and my own experience induces me to regard
his “ spermatia ” as parasitic bodies, in no way vitally connected
with the fronds on which they occur. In the only species, S, Grevillii,
in which I have noticed conjugation, and the formation of sporangia,
the process is analogous to that throughout the Diatomaceæ ;
and the same analogy holds good in the allied genera CoUetonema
and Encyonema. In all, conjugation takes place within the frond,
or after the dissolution of the mucus-envelope, and follows the usual
course.
The characters hitherto chiefly relied upon in specific descriptions
of the Schizonemata also appear to me deficient in permanency, and
to lead to a needless multiplication and confusion of species. Neither
size, colour, nor ramification is sufficient to distinguish fronds that
vary in all these respects with age, locality and exposure. A remark
on this subject, made by M. De Brebisson in his ‘ Considerations sur
les Diatomées,’ 1838, p. 8, is deserving of attention, viz. :—“ La plupart
des espèces doivent être revues de nouveau, et étudiées avec
plus de soin en ce qui concerne les frustules. On n’a pas tenu assez
compte de leurs formes exactes ; les espèces ont été seulement
établies d’après la disposition et la couleur des frondes ou filamens,
caractères trop variables.” Nothing can he more ju s t ; the size,
form, striation and structure of the frustules supplying far more certain
elements for specific distinctions than the characters hitherto
too often relied upon. Acting upon the above suggestion of M. De
Brébisson, I divide the species into two sections ; those of the first
having frustules firmly siliceou.s, and fronds iu consequence some