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All in the family....
"Is Najas related to Lagarosiphon madagascariensis and/or Egeria densa?
They seem to have the same leaf-vein-structure.."
The answer to this depends upon what you mean by "related". As Tom has
pointed out, many features of a plant are considered when determining its
nearest relatives. Superficial physical similarity can often be the result
of unrelated plants having to deal with the same sort of environment. Flower
(and fruit) morphology is usually much more important than leaf vein
structure, but there are many characteristics which are considered,
including, in recent years, the structure of the plants's DNA.
In practical terms a lot also depends upon the use and circumscription of
the "family" unit, i.e. where the limits are drawn to decide what is "in"
and "out" of any particular family. For many plant families, this is still a
matter of opinion (and a subject of debate) amongst botanists.
Both Lagarosiphon and Egeria are considered to be members of the family
Hydrocharitaceae, a group of aquatic "herbs" (plants that don't form woody
stems). Najas has at times been included within the Hydrocharitaceae and at
times placed in its own, separate family Najadaceae. There isn't any doubt
that Najas is "different" from the other members of the Hydrocharitaceae,
the only discussion is if it is "different enough" to warrant placing it in
its own family, separate from the rest of the members of the family. Its a
classic case of "splitters" vs "lumpers". Some botanists prefer to have a
smaller number of loosely defined families (easier to memorize), others
prefer to have a larger number of more tightly defined families (more
precise). But even amongst the "lumpers", Najas is usually placed in a
separate subfamily "Najadoideae" (family names usually end in -aceae while a
subfamily name ends in -oideae ) within a larger, more inclusive
Going back one step up the tree, (Kingdom, Division, Class, Order, Family,
Genus, Species), all three genera are considered to be members of the Order
Alismatales (and this is the same whether Najas is considered to lie within
the Hydrocharitaceae or is placed within Najadaceae). So there is SOME
relationship between all three genera (i.e. they ALL share a common ancestor
at some time in the distant past).
But that is also akin to saying that everyone is related to the Queen of
England, if you trace your family tree back far enough.
(who, apparently, is related to the royal family of France, via an
illegitimate coupling between a stable hand and one of the "Louis"
daughters...alas, the money and the jewels never made it this far...)