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The Essex Naturalist
their identifications. I have, however, drawn up a set of annotated drawings of the
genera, together with illustrated keys to the taxa most likely to be found in eastern
England, as a separate pamphlet, that will, I trust, avoid some of the ambiguities.
They should however be used in conjunction with the BSBI Handbook. These
keys for example purposely avoid using the degree of encrustation as a definitive
character as it has been found to be variable in Essex material.
Morphology
Stoneworts are formed from gigantic, cylindrical, multinucleate-cells, up to
several centimetres in length, up to several millimetres in diameter, and often
containing several thousand nuclei. The cytoplasm, containing the nuclei and
chloroplasts, forms a thin, hollow cylindet, surrounding a large central vacuole,
renowned for its easy observation of cytoplasmic streaming. In the simplest
forms, as in the genus Nitella, these giant tubular cells are arranged as an end to
end row, to form the axis of the plant, each separated by a ring of short cells
forming a NODE, from which whorls of cylindrical cells, BRANCHLETS, then
arise like the spokes of a half-open umbrella. These may be divided several
times, with either two or three equal-length segments at each level, the final
"fingers" being known as DACTYLS. Every-so-often the main axis branches, the
new branch arising from a nodal cell. Normally, Nitella produces a pair of
branches per node, whereas the genus Chara produces just one. A BRANCH
can grow on indefinitely, giving rise to whorls of branchlets, but the
BRANCHLETS themselves have limited growth, and terminate in the dactyls,
which usually have a definite shape that is often useful as a character for
identification. Nitella taxa differ from each other principally in their overall
dimensions, the number of cells per dactyl and the extent of branching. They
are attractive plants, generally unencrusted, and resemble shining rods of
translucent bright-green glass.
In the otherwise more complex genus Chara, the whorls of branchlets remain
like the ribs of an umbrella and are never branched. In Chara, however, small
bud-like cells arise at the nodes, one above and one below each branchlet base,
and grow downwards and upwards respectively until they meet in the middle of
each internode. Thus forming a sheath or CORTEX of tubular cells, equal in
number to the branchlets, around each of the axial cells. The rows of cortical
cells twist in a shallow tight-handed helix around the axis, so that under the low
power of the microscope, or under a x20 handlens, the surfaces of the main axes
appear to be helically striated. At semi-regular intervals these tubular cells cut
off a nodal cell, and then continue to grow on down, or up, the axis. The nodal
cells usually then divide twice to give three cells. These may remain inert or
may develop into singled celled spines. Alternatively, the two lateral cells may
elongate and form an additional secondary row of tubular cells either side of the
primary, spine-bearing rows. In many cases the adjacent secondary row cells
arrange to mutually alternate in position, so that each primary spine bearing row