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Ludwigia classification
I have two species of similar plants which I'd like to ask a bit of help in
identifying. For the moment, I'm assuming they are some species of
Ludwigia.
Variety a) has leaves about 3-4 cm in length and vary in width from
~5mm in moderate light to 10mm in strong light. The leaf tip is pointed and
the edges are undulate. The color is olive green but in good light becomes
much redder (olive-red-brown?) The nodes are about 1 - 1.2 cm apart,
the stems slender ~1.5mm. The leaves are widest at 1/2 to 2/3 toward the
tip and taper toward tip and base (oblanceolate). The larger leaves are
widest about 2/3 of the way toward the tip. The petiole (leaf stem) is not
a distinct feature; kind of a thickening of the central leaf vein; the leaf
surface starts almost as soon as the stem branches from the stalk internode
but is not so closely attached to the stalk as L arcuata which has no
petiole to speak of. The tip of the leaf is pointed (acuminate) but
not at an acute angle (about 60-45 deg?).
Variety b) has much closer nodes; 1 - .5 cm; the leaves are shorter
~2.5 - 3.5 cm but broader; .9 - 2 cm; the broadest part is about 2/3 toward
the tip and the edges are quite undulate too. The coloring is similar to
the other variety but never reaches the deep coloration of Alternanthera
reineckii. The leaves on this plant do not show the same variability in
shape as a) retaining the approximate ratio between length and width. The
size of the leaves however varies increasing as the plant is able to
establish its root & vascular system and with overall improved conditions.
The leaf tip is also pointed but at an angle of about 70-90 degrees.
The stem is somewhat thicker than a) ~2mm.
Both plants are similar on coloration; I underside of the leaves (the
bright light ones) has an iridescent red coloring. This characteristic
of the dark underleaf surface is very marked in L repens. The stems
are green but reddish toward the tops (better light or more Fe or
as the plant becomes better established) The stems do not have the
consistent red color of L arcuata.
They both resemble the coloration of Ludwigia arcuata when its leaves are
close to the surface in good light. Also like L arcuata the stems are
nearly vertical and branch if the top is cut. The leaves are arranged in
pairs which alternate at 90 degrees. The pointed leaf tip feature esp.
where the leaf converges obtusely and then forms a very sharp elongated
tip is an evolutionary adaptation of land plants designed to assist the
plant in shedding rain drops which in tropical climate would otherwise
encourage fungii and other parasitic growth. Aquatic plants with this
characteristic are commonly those which have evolved from terrestrial or
amphibious forms such as Ludwigia. L arcuata (or at least what I have)
has a very round leaf tip although the T.F.H Aquariums Plants (Windelov)
describes it as having a pointed tip.
Possibilities include L glandulosa, L mullertii, L palustris or
(stretching) L brevipes although they do not resemble some pictures of
the latter plant which has quite narrow (pointed) leaves.
Unfortunately, good quality color photographs of a large variety of
Ludwigia are hard to find. L mullertii is described in Aquarium Plants
(Horeman) as a horticultural variation of L repens with a redder underleaf.
(How do you get redder than that?)
The closest picture I've found so far is in Handbook of Tropical Aquarium
Fishes (Axelrod & Schultz), the b/w picture on the bottom of p 117:
L palustris var. americana which resembles the strong light leaves of
type a).
Can anyone help out with classification or point us to some references
on this family? Thanks! We should figure out a way to trade or publish
pictures that doesn't involve www. It would be great if we could do
it in The Aquatic Gardener (journal)!! (even only one page an issue)
Steve