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282
tyge \v. bücher
heterocysten- and dauerzellenlose Trichome erzeugen». He supposes that
P. constrictci, which occurs in i)ottom mud with a low oxygen pressure,
generali}* fails to form heterocysts. A closer study of the paper of
Canna-baeus hbwever. shows that only Anabaena variabilis produces such
hormogonium-like short trichomes without heterocysts. and these short
parts never develop turtlier. Curiously enough Geitler does not mention
some experiments made by Cannabaeus (I.e. p. 21) which show that
a lack of oxygen enlarges the heterocysts in Anabaena laxa, variabilis
and oscillatorioides.
In this connexion the structure and behaviour of a new species
Pseudanabaena biceps may be of special interest. This species is found
in bottom mud from the above-mentioned lake together with sulphur
bacteria and yellowish green Oscillatoriae in abundance. In this species
the young cells are rounded and may remind of those described by
Koppe in P. constricta. Furthermore constrictions occur in dividing cells.
The new alga exliibits spontaneous, rather vivaceous creeping or gliding
movements. It bas a very characteristic appearance. The terminal cells
are möre or less pointed and nearly always contain a ring of gasvacuoles
(pseudovacuoles) or one large ringshaped gasvacuole in their distal
parts. Sometimes one or two supplementär}* small gasvacuoles occur
in the tips of the terminal cells (Figs. 7. 8, 10). In longitudinal sections
the large gasvacuoles appear as two separate reddish bodies or as two
light, refractive bodies. which particularly inwards liave a broad reddish
contour. In the other cells of the trichomes the gasvacuoles may either
be missing or they may occur as minute reddish disks close to the
transversal walls (Fig. 1). The new terminal cells of newly formed
hor-mogones generali}* liave no gasvacuoles. Sometimes, however. very
small gasvacuoles are observed near the ends of such cells (Fig. 2),
suggesting a change of the disklike bodies into true gasvacuoles. Thus,
the formation of gasvacuoles is localized to the ends of the cells and
within whole trichomes the development of the large gasvacuoles is
sharply localized to the parts immediately behind the tips of the two
terminal cells. A similar localization of gasvacuoles to terminal cells
bas been found by Geitler in Oscillatoria acuminata, hut within these
cells no localization was established in this case.
Most observations were made in preparations made by some mud
being mixed witli water and sliut up under a large cover glass provided
with a rim of vaseline. Pseudanabaena biceps would live several weeks
in such preparations. In some of the preparations all gasvacuoles
dis-appeared and simultaneously the chromoplasma was withdrawn,
apparrently forming openings filled with centroplasma (Fig. 9). Any
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