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Fig. 8. Growth rate of tufa on tablets exposed at L´ˇky and H´j sites.
found at the Z
´
zriv
´
site (Fig. 14a, b). The most
mature form of the bushes reaches the height of
0.8 mm and width of more than 1 mm. It comprises
several branching tubes, thus the whole bush sys-
tematically widens upward. Its peripheral zones
overhang and the whole structure obtains an
umbrella shape. Such bushes overgrew the tablets
exposed between August 2002 and October 2003.
Their initial forms appear on tablets exposed sea-
sonally. They have a form of a single tube, only
30 mm in height, oriented perpendicular to the sub-
strate tablet, or small bushes with a limited number
of ramification (Fig. 14c). The tubes forming the
bush occur separate or coalesce with the neighbour-
ing ones. The form of twin-tube was particularly
common in the spring of 2003 (Fig. 14d).
The rounded, ovoid cells of algae located at the
tube tips were observed under SEM in lyophilized
samples (Fig. 14b). Organic mucus was detected
between the cells and the calcitic tube. The internal
diameter of tubes is constant and varies between
17 and 20 mm. Each tube is built of a single sparry
calcite
crystal
composed
of
small
paralel
subcrystals.
The spatial arrangement and diameter of tubes,
as well as the cell shape prove that the structures
are composed of unicellular desmid alga Oocardium
stratum. This alga is one of a few freshwater species
intimately connected with calcite precipitation
(Pentecost 2005, 164 - 165). It was characterized
by
Wallner
(1935a,
b)
from
Germany,
who
described
the
ecology
of
the
species
and
the
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