i
imperfect birth of the germ cells just described is repeated at the transition
to the third, and mostly even to the fourth generation, so tliat
little arborescent groups are produced with twice or thrice-repeated umbellate
ramification, till at length the cells which form the outermost
nmbellules scatter out their germ cells, which, after a short swarming,
fix themselves again to be developed into ramified stocks of new
families”—Brmm Rejuvenescence, p. 187.
Plate XV. a, h, young cells ; c, commencement of the first generation
of daughter-oolls ; d, further progress of the first generation of progeny ;
e, second generation being evolved from the first ; /, old plant evolving a
third generation X 3Ü0, after Braun; g, zoogonidia.
Sub-Family 7. P e d ia s t e e æ .
Cænobium discoid, plane.
For other features of this sub-family see the characters of the genus,
which is the only one at present comprised within it.
G e n u s 29. P e d ia s trum . Meyen. (1829.)
Cænobinm plane, frond-like, discoid, or stellate, free swimming,
formed of cells in a single, rarely in the centre in a
double stratum, continuous, or with the cells here and there
interrupted, perforate or clathrate. Cells polygonal, central
entire or slightly emarginate, those of the periphery entire or
two-lobed, the lobes wedge-shaped, either simple or two-toothed,
sometimes elongated into a born. Cell-contents green, homogeneous
at first, then granular.—Babh. Alg. E u r. iii. 69.
Formerly this genus was included in Desmidiaceæ, but the knowledge
of its life history has shown that it has no relationship with the Con-
iugatæ. Brann illustrated the development of one species (Rejuvenescence,
PI. III.), and we have reproduced some of his figures (on PI. XVI.)
Fig. 1 is an old disc, in great part emptied by the birth of gonidia.
Several of the empty cells exhibit a cross slit, through which the contents
have been discharged. The order in which this emptying took
place is indicated by the letters a, b, c, d, e. One cell is in the act of
discharging the gonidia, these having in part entered the projecting portion
of the hernia-like vesicle, formed by the swollen innermost layer of
the membrane of the mother-cell, in part still remaining in the internal
cell cavity. Three other cells still possess their perfect contents in
different conditions. Two of them are filled by sixteen extremely closely
crowded gonidia, only half of which are visible, as they form a double
layer. The third nnemptied cell is in the actual transition to the formation
of gonidia. I t exhibits the first division of the contents iuto two
halves, one of which already appears halved again. Fig. 2 is a new-horn
family immediately after the birth. The innermost layer of the mother-
cell has wholly emerged from the old cell, as an extremely thin vesicle,
enclosing the gonidia, the gonidia in the interior moving actively. Fig.
3 is the same family, as seen from the upper surface. Fig. 4 is the same
family, a quarter of an hour after birth. The gonidia, now at rest, have
arranged themselves in a plane disc. Fig. 6 is the surface of the same
family at the same stage. Fig. 6 the same family one hour after birth.
The emargination of the cells has proceeded further. Fig. 7 the same
again, but four hours after the gonidia ceased to move. The emargination
of the border-oells has passed into the formation of horns. The cells are
not even yet closely connected together, bnt exhibit spaces between
them, so that in this stage it resembles P. pertusum. Not until the
second day do the cells become closely applied together ; the horns acquire
their proper shape and length at the same time. All the figures
are magnified 400 diameters.
The number of cells which enter into the composition of a single disc
vary in the same species, so that it cannot be accepted as a character.
The arrangement and limit of species adopted are those proposed by
Braun (“ Algarum unicellwrum,” 1855).
Plate X V I. figs. 1 to 7. Development of Pediastrum, after Braun; 8
a, h, zoogonidia.
Section 3. d ta c tjn ium . Braun.
Cells of periphery emarginate or bilobate, lobes entire.
P ediastrum se lenæ a . Kutz. Babh. Alg. i l l .13.
Cænobium prbicular, entire, formed of 8-16 (rarely 31) cells.
Cells of periphery narrow, lunate, acutely lobed ; cells of disc
slightly excised, central one five-angled ; substance firm, rather
thick.
Siz e .* Cænobium '028--085 mm. diam.
Braun Unicell. Alg. p. 83. Kalfs Desm.t. 21, f. 5.
Pediastrum Napoleonis, Ralfs Ann. N. Hist. xiv. (1844) t.
12, f. 6.
Pediastrum elegans, Hassall Alg. t. 86, f. 19.
Pediastrum lunare, Hassall Alg. t. 92, f. 8.
In bogs, moor pools, &c.
Plate X V I . fig. 9. a, 8-celled cænobinm ; b, 16-oelled cænobium X
400 diameters ; c, marginal cell.
P ediastrum an gu lo sum . Ehr. Babh. Alg. ill. 73.
Cænobium orbicular, oblong, or subreniform, continuous,
composed of 8-16-82-64 cells. Cells all even, angular, those
of the periphery truncate at the base and dilated upwards, more
or less deeply notched in the middle, the lobes obliquely truncate,
outer angle very shortly apiculate, inner one ending in a
shorthorn. Central cells 5-6 angled, slightly vepand in front,
marked with a small transverse oblong pallid spot.
S ize. Cænobium-12 mm. diam. ; c e lls -019 diam.
Hass. Alg. t. 86, f. 14. Ralfs Desm. t. 31, f. 11 a b.
Braun Unie. Alg. p. 84.
Pediastrum Boryanum, Ralfs Ann. Nat. Hist. xiv. (1844) t.
12, f. 7, upper.
Pediastrum excavatum, Hassall Alg. t. 92, f. 6.
In bogs.
Plate XVI. fig. 10. a, marginal cells ; b, 16-celled cænobium ; c, 8-celled
cænobinm X 400 diameters.
* The size must depend on the number of cells of which the ooenobium
is composed ; hence throughout this genus the dimensions given must only
be accepted as approximate.