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The Essex Naturalist
have a sheathing cortex of tubular cells, and they have nodes. These bear a
single ring of spine-like BRACT cells, usually graded in length, longest on the
inside, shortest on the outside. Finally, if you pull out a hunch of Stonewort from
a pond and it smells unpleasantly of hydrogen sulphide, its likely to be a Chara.
In some countries Chara and Nitella species are introduced to ponds and lakes
because they are supposed to deter mosquitos.
In Tolypella, the only other charophyte genus likely to be encountered in
Essex, the branchlets are formed from linear filaments of cells, that unlike
Nitella, are unbranched, and the reproductive organs are surrounded by globular
bunches of branchlets, looking like magpie's nests in a tree.
Reproduction
Stoneworts produce quite distinct male and female gametangia. The oogonia are
usually bright green and just visible to the naked eye. The single ovoid egg cell,
packed with starch, is enveloped by a spiral cage of five sterile, tubular cells,
which become thick-walled on fertilization. The resultant oospore (0.4mm to
1.1mm long according to species) being said to be highly resistant to desiccation
and long lived. Since algae are said to differ principally from other
photosynthetic plants, in having no multicellular covering to their gametangia,
this rather puts Charophytes out on a limb. This, combined with their domed
apical cell, which cuts off a row of cells that alternately differentiate into
elongating axial and short nodal cells, has convinced many people that they were
worthy of their own phylum, the Charophyta. The current view, based on ultra-
structure and enzyme studies, is that they are an advanced group of chlorophyte
algae, close to the evolutionary link between green algae and the first land plants,
the bryophytes. The distinctive chemically resistant oospores however, fossilize
readily and provide us with a more or less unbroken record of their past presence
on earth. Nitella and Chara oogonia are identical in basic structure, except for the
apices of the five sterile filaments, which are gathered together at the apex to
form a crown or CORONULA. In the genera Chara, Nitellopsis and
Lamprothamnium a single cell is cut off from the top end of each of the five
filaments to form the coronula, whereas in Nitella and Tolypella, two cells are cut
off, giving rise to a two-tiered crown. In Nitella the oospore is somewhat flattened
and oval in section whereas in all the genera it is round in transverse section.
The spherical antheridia are usually much more conspicuous, due to their
bright colour, ranging from whitish through yellow to orange to red, as they
mature and accumulate carotenoides, though at 0.15 to 1.5mm diameter
(according to species) they would otherwise be inconspicuous. The antheridia
are exquisite objects and well worth squashing under a coverglass to see their
complex components. The spherical multicellular antheridium is lined by eight,
bright-orange, flat, shield-cells that are rounded triangular in shape, and each
lobed into about 12 lobes. These fit together to form a hollow sphere. Projecting
inwards from the centre of each shield is a bright green tapering, cylindrical cell