ALGÆ LIGURATÆ.
Fam. XIX. DESMIDEÆ.
Char. Algæ rarely filamentous, bipartite, o f a figured outline,
higlilg mucous.
W e have arrived at length at the consideration of a most
nteresting group and family of freshwater Algæ, the essential
characteristics of which are the beautifully figured forms
and bipartite composition of the fronds of the numerous
species of which it is composed.
The Desmideæ have been the subject of considerable discussion
respecting their animal or vegetable nature : the
question is now, however, all but settled in favour of their
vegetahility. Amongst the advocates of their animal nature
may be mentioned the names of Ehrenherg and Mr. Dalrymple
: the observations of the latter, however, were confined
almost exclusively to the genus Closterium.
Ehrenherg rested the claims of the Desmideæ to animality
upon the mode of their increase by self-division, a
method of multiplication which it is now known belongs to
all the Algæ, and which, therefore, cannot be allowed to have
any weight in the decision of the point in question. Mr. Dalrymple,
however, in his memoir on the Closteria*, adduces
several other reasons for regarding them as animals, two only
of which apply at all to the other genera of Desrrddeæ, the
remainder being alone applicable to Closterium : these are,
first, the fact tha t the inner delicate membrane, which lines
the outer symmetrical, and usually crescentic covering, con*
See “ Annals of Nat. Hist.” vol. v. p. 415.
tracts forcibly on the application of certain reagents, the
action of which cannot be considered as purely chemical;
and second, tha t iodine does not detect the presence of starch
in the cells. The former observation is equally applicable
to the tender membrane which lines the cells of true Confervoe,
while the latter remark is wanting in accuracy, for
it is only when the Desmideæ are very young, and before
the contents of the cells assume a granular appearance, tha t
iodine does not make manifest the presence of starch ; subsequently,
however, when the endochrome has become granular
and vesicular, starch is readily detected by means of Iodine,
it turning the large vesicular granules of a blue colour.
Meyen advocated the vegetahility of the Desmideæ, and
was the first to detect the presence of starch in the cells, but
the accuracy of his remarks has been doubted ; they are now,
however, fully confirmed by Ralfs, Jenner, and the majority
of recent algologists.
The Desmideæ, I conceive then, are to be regarded as
vegetable productions, agreeing with the Confervoe proper, of
whose vegetahility there can be no question, in the following
particulars.
1st. Probably in the elementary composition of the cellular
tissue.
2d. In the undoubted presence of starch in the cells.
3d. In the multiplication of cells by division.
4th. In their reproduction.
The reproduction of this family has been stated by some
writers to be threefold. I am disposed to think, however,
tha t there is but one essential and true mode of reproduction.
The three methods by which the Desmideæ are stated
to be multiplied are, first, by the division of the cells ;
second, by bodies analogous to zoospores ; and third, by
true spores, formed as in the Conjugateoe, by the union,
intermingling, and condensation of the contents of two
distinct individuals. To the family Conjugateoe indeed the
Desmideæ would appear to be more nearly related than to
any other. The first cannot be regarded as a true mode of
reproduction ; it is but a continual growth, repetition, and
z 2