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var. C. dimozphus. R aih . Alg. m ., 63.
Cells acute, 4-8, placed evenly in a single row ; inner cells
fusiform, outer externally lunate.
Scenedesmus dimorplms, Ralfs. Ann. Nat. Hist xv., p. 403,
t. 12, f. 5. Brit. Desm. p. 191, t. 31, f. 13. Hass. Alg. 393,
t. 92, f. 13. >
Formerly the typical form and its two varieties were regarded as three
separate species, but there scarcely seems sulHoient reason for thus retaining
them. Ralfs wrote of them long ago, “ When the cells are nearly
uniform A has some resemblance to S. dimorphus; but in the
latter the cells are more slender, never ventrioose, and are arranged
quite evenly side by side. I t is more difficult to distinguish S. acutus
from S. obliqnns, and I am far from certain that Bhrenberg erred in
uniting them. The principal distinction is that in 8. acutus the cells
form only a single series, which is nevertheless irregular, on account of
the alternate projection of the cells in opposite directions. In S. ohliquus,
on tho other hand, the cells by division form two distinct rows, which,
after separation, become two fronds.” These characteristics are better
shown in Ralfs’ figures than in our own, which are more intermediate,
and show an evident approximation to the typical form.
Plate X I I I . fig. 6. a, cells of the typical form ; cells of the variety
dimorphus; c, of the variety obligmts. All magnified400 diameters.
Scenedesmus a n te n n a tu s . Breh. Balf. Desm. t. S5, f. 27.
Cells fusiform, 2-4-8, joined in a single or double series, all
somewhat curved, usually ventrioose, cuspidate at each extremity,
the apices bearing a hyaline globule.
S iz e . Cells '0025-'0035 mm. diam., '013 mm, long.
Rabh. Alg. iii. 63. Archer in Pritch. Infus. p. 753.
In pools.
“ Scenedesmus antennatus resembles 8. acutus in form, and also in the
arrangements of the cells; but is distinguished from that and every
other species by having the attenuated points tipped by minute globules.”—
Ralfs. r r J 5
At the time the above was written it had not been found in this
country, and is still the rarest species, if its specific identity can be
maintained.
Plate X I I I . fig. 7. Cells magnified 400 diameters.
B. Cells armed.
Scenedesmus q u ad ric au d a . Breb. Balfs Desm. 190, i, 31, /. 12.
Cells oblong-cylindrical, each extremity obtusely rounded,
2-4-8, narrowly united, either in a single or double series, all
straight, the outer cells a t each end (and rarely some of the
intermediate ones) armed at each extremity with a recurved
spine.
S iz e . Cells -OOSS-’Ol mm. diam. to -022 mm. long.
Rabh. Alg. iii. 65.
Scenedesmus quadricaudatus, Ralfs Ann. Nat. Hist. xv. p
402, t. 12, f. 4. Hass. Alg. 392, t. 92, f. 12. Jenner FI. Tun!
Wells, p. 200.
Ill standing water.
The commonest of British species. A variety has been described
which differs only in being entirely destitute of bristles. We can confirm
Balfs in his observation that the species of this genus frequenUy
make their appearance in clear water that is kept m glasses or bottles
and exposed to the light. He says that he has repeatedl^y noticed the
appearLce of S. aeutus var. obliquas in bottles containing Desmidieæ, and
sometimes its rapid increase so as to ontnumber its companions, hpeoi-
mens obtained in this manner, he adds, are frequently more or less
distorted. In little aquaria the present species often becomes a nuisance
from its profusion. ^ -i- l
Plate X I I I . fig- 8. Cells magnified 400 diameters.
Suh-Faniily 5 . H y d e o d i o t t b æ .
Individual cells oblong-cylindrical, united into a reticulated
saccate ooenobium, all fertile, some producing macro-
gonidia, which join themselves into a cænobium withm the
mother-cell, others producing microgonidia, which are tur-
nished with two vibratile cilia and a lateral red spot ; these
escape from the parent-cell, and, after a brief motile period,
subside into protoooocoid, thick-walled spores.
Genus 26. HYDK.ODICTYON. Both. (1800.)
Characters the same as in the sub-family.
“ The genus Hydrodictyon comprises, as far as known, bnt a single
species, 4 i c h is common to North America and Europe. I t grows m
great abundance in the neighbourhood of Philadelphia especially m the
ditches and stagnant brick ponds in the low grounds below the city
known as the ‘ Neck.’ There it very frequently forms floating masses
several inches in thickness, and many feet in extent, so that with the aid
of a rake it could be gathered by the bushel. When thus in mass the
colour is very generally dingy and yellowish, although the fronds, when
in active vegetative life, are mostly of a bright, beautiful green The
plant is in greatest profusion in June and July, after
gradually disappears, until in the autumn it is scarcely to be found, but
farly in the spring it reappears. The very young fronds are minute,
ovab cylindrical, fllmy-looking closed nets, with the meshes not appreciable
to the eye ; when growth takes place the fronds enlarge, until
finally they form beautifnl cylindrical nets, two to six inches in
with their meshes very distinot, and their ends .dosed; 1“ * e bright
sunlight, they, of course, by virtue of the life functions of their chlorophyl
liberfte oxygen, which, being free to the interior of the net 'te exit
barred bv the fine meshes, collects as a bubble in one end of the oylindei,
and buoys it up, so that, the heavier ends sinking the net is suspended
as it were, vertically in the water. I know of few Grmp
more beautiful than a jar of limpid water with masses of these little nets
hanging from the surface like curtains of sheen m the bright snnhghk
A few cells collected in the fall or early spring, if put into a presemn
jar, and the water occasionally changed, will multiply, and m a little while
become a source of frequent pleasure to the watcher.
“ As the fronds increase in size they are always in some way or other
broken up, so that, instead of being closed cylinders, they appear as
simple open networks of less or greater extent. The extreme length to
which the frond attains is, I think, very rarely over twelve inches, with
meshes of about a third of an inch in length. The construction of the
frond is always the same. It is composed of cylindrical cells united end
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