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340 D E SM IDEÆ .
multiplication of cells; and while it effects the increase of
the individuals of a species, does not provide against the extinction
of that species when it shall have reached the te rmination
of the brief existence allowed to it. Moreover, the
cells resulting from the bisection of other primary cells have
no periods of juvenescence and growth ; they arc produced
at once fully developed and perfect in size and organization.
I t is the nature, on the contrary, of a true reproduction, tha t
the bodies or organs by which it is effected should be at first
minute, and subsequently pass through successive stages of
developement. The second method is assuredly the usual and
legitimate mode of reproduction, viz. tha t by bodies analogous
to zoospores, while in the third the organs resulting from the
union of two individuals are probably to be regarded in the
same light in which Agardh viewed the similar bodies of the
Conjugateoe, viz. as receptacles in which the zoospores are
stored, and destined, as I think, not for immediate^ but for
future use, that they are in fact hibernacula, designed to
preserve the contained propagules until the vicissitudes and
rigour of winter shall have passed away.
The formation of sporangia has been noticed to occur m
nearly all the genera of the family of Desmideæ, and it is
probable that it occasionally occurs in all of them. By
Ehrenherg it has been noticed in different species of the
genus Closterium, by Brebisson in Desmidium, by Ralfs in
his genus Tetmemorus, and in Staurastrum. In the Cylin-
drocystis Brehissoni, a production placed by Meneghini
amongst the Nostochineoe, but which seems to me to belong
to Desmideæ, union of the cells has been observed, and it is
most probable tha t this union is followed by the formation of
sporangia.
So much for the reproduction of the Desmideæ: a few
words may be added upon their growth. I t has already
been stated in the definition of the famfiy that certain species
of Desmideæ are filamentous. The filaments of these increase
in length though not in number by the continual
division of the cells, as do other filamentous Algæ-, but
in the Desmideæ, which are not formed of filaments, the
D ESM ID IUM .
growth of cells is different, and approaches rather more
closely to a true reproduction, which, however, it is not.
Each of these Desmidia consists of two portions, or cells, frequently
contracted at their points of junction. These segments,
when they have arrived at maturity, separate from
each other, and from the open extremity of each a little
mucous pouch extends : this imperceptibly increases in size,
and finally assumes the form and characters of the originally
formed segments. This mode of formation of cells is highly
curious, and it is one which may be accounted for by reference
to the form and constitution of two portions of those
Desmideæ which present it.
Another fact, not as yet alluded to, has been advanced in
support of the animality of the Closteria by Mr. Dalrymple :
this is the presence in each extremity of Closterium of a
distinct organ or vesicle which contains a number of active
and revolving molecules. B u t moving and revolving particles
are met with in many undoubted Algæ, and, therefore,
their presence in Closterium cannot be considered as decisive :
moreover, no such organ presents itself in any other genus
of Desmideæ, the proofs of the vegetahility of which may
be regarded as decisive, and to which the Closteria are too
evidently related for the idea to be entertained that the one
are animal a n d /h e other vegetable productions.
The Desmideæ are found in old boggy pools, the waters
of which are not periodically dried up, but which are permanent
for several years. They do not usually float on the
surface, but are found in cloud-like masses near the bottom.
They are best removed on pieces of linen, on which, if
moistened regularly, they may be preserved unaltered for
several days.
Section i.
Frond filamentous.
51. D E SM ID IU M Ag.
Char. Frond filamentous, simple, spirally twisted, fragile,
articulated and angular ; mature cells bipartite ; angles
z 3