The Essex Naturalist
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is separated by a single, composite secondary row, without spines. Very often in
Chara vulgaris, for example, narrow tramlines of primary row cells grow all the
way until they meet at the centre of an internode, while the secondary row cells
lag behind, and subsequently push their way along the gaps inbetween the
primary rows, like fat fingers. In other species, both secondary rows grow at the
same rate as the primary rows, and so form two secondary rows without spines,
between each spine bearing row. This may all sound very confusing but should
become clear from the diagrams in the pamphlet.
Whichever of these three types of development occurs, is genetically
determined and highly conserved in any one taxon. From the observers point of
view, it is therefore very important to look at a new specimen initially under a
microscope and check for spine cells, which in some species simply remain as
small round or oval cells, whereas in others they develop into groups of one, two
or three projecting spines. Very often they develop on the young, short
internodes, and then drop off as they elongate. Having found these, one needs to
check whether the spines or spine cells occur along every row, every other row or
every third row of the cortex. It is always best to establish this pattern by looking
either just below or just above a mature node, as if the cortex is incomplete in the
middle of a node, it may be difficult to decide. Note (as few artists have!) that
spines point downwards, on the primary cortical rows growing downwards from
the node, and point upwards on primary rows growing up from the node. In the
case of many Chara species the plants become encrusted with a crystalline layer of
calcium carbonate in the form of calcite, making it difficult to observe surface
features. In calcium-rich waters, this is achieved by active secretion of OH' ions,
resulting in CO32- saturation of the water around the plant, and consequent
precipitation of calcium carbonate on its surface. Half an hour or so in neat
vinegar, however, is usually sufficient to allow the acetic acid to decompose the
carbonate to carbon dioxide bubbles, thus revealing the structure of the cortex.
Having decided whether spines are borne on every cortical cell row, every
other or every third, - one then needs to determine whether the rows that bear
the spines are prominent, thus forming ridges, with the secondary rows narrower
and sunken, or whether the spine bearing primary row is sunken in a groove
with prominent secondary rows forming the ridges. This is a more plastic
character than the number of secondary rows and in some cases even cutting a
section may reveal only a slight difference.
In Chara, Lamprothamnium, and the one genus not represented in Britain,
Lychnothamnus, either one or two potential circlets of spine-like cells surround
each node. With two upper and two lower STIPULODES, as they are called, per
branchlet, in Chara, or just one downwardly pointing stipulode per branchlet, as
in Lamprothamnium. In some Chara taxa, these cells may be minute and globular,
and one row may even be suppressed. In most Charas however they point
upwards and downwards as paired spine-like cells, the upper row usually being
better developed than the lower. Just like the main axes, the branchlets in Chara