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[APD] Re: cuticles etc...

Steve Pushak said:

> These leaves are typical for a terrestrial succulent. We should probably
> classify it as slow growing. "Growing well" is relative.

Excuse me, but "Growing well" is _not_ relative. "Slow growing" certainly
is. It is slow only if compared to something else. But as long as it grows,
it grows.  If we are going to talk relative than yes, it is slow growing
when compared to Hygrophila polysperma, as any succulent would be. But it's
a hare when compared to Anubias.

> Perhaps it is using stored reserves during this submerged time?

I believe I've said that it germinates underwater. It might use up its seed
reserves for some time but these run out eventually. In my tanks I have kept
them growing submerged for months.

> Can we tell if it is photosynthesizing submerged?

If it is growing it must be photosynthesing. No? If it is photosynthesing it
must be getting its CO2 from somewhere.

> If the leaves are waxed and the surface area to mass ratio is low as is
> the case for a succulent, then it would have a hard time to absorb
> enough CO2 from the water. Perhaps it is utilizing carbonate instead!?

Maybe. I don't know. I know it can utilise the CRASSULAcean Acid Metabolism
(CAM) photosynthetic pathway. That would make CO2 usage more efficient.

> Another possibility is that it is getting CO2 underground; but I think
> this is unlikely. A substrate high in CO2 would be strongly reducing &
> this would not be a good habitat for a plant with limited oxygen
> producing potential.

The substrate can be just a few millimeters thick in nature, not much chance
for reducing conditions.

> Do you have this plant growing submerged for a long period of time? Does
> it require CO2 fertilization to survive?

Yes in my tank. No it does not need added CO2.

> Does it cause pH to rise over time when CO2 is not supplemented? This
> would indicate carbonate utilization.

I have not checked. But it would need a dedicated tank.

> How do we know how thick the epicuticular wax layer is? Are you judging
> by appearance or analysis/measurement of the layer?

I do not know how thick it is. I don't need to. It is enough to live
terrestrially in dry conditions. I know it is there because otherwise the
plant would dry out when exposed as other aquarium plants I keep do.

> I could not find any picture of Elatine gussonei.

I don't think you will on the web. But I'm sending you a couple of close up
photos of Elatine gussonei and Crassula vaillantii  in water, by e-mail. I
will send them to anybody else who is interested if they send me a private
e-mail asking for them.

>I don't think there is any way for a
> plant to loose its waxy cuticle once it has it nor to rapidly form one
> on a flimsy underwater leaf.

I agree.

What I think is worrying you is how come a plant with a cuticle on its
surface is respiring? Well what if it has stomata? I know that most aquatic
plants do not. They do not need them, since they don't have a cuticle. But
if a plant retains its cuticle (because of its habitat) would'nt it retain
its stomata? And stomata can open underwater. The surface tension and/or the
positive pressure (due to O2 prodn) would prevent the leaves from flooding,
which means that CO2 transfer and oxygen release can still take place,
albeight at a slow rate. But remember that the plant is a CAM user, and
therefore efficient. I think I read an article by Tom Barr somewhere re CAM
photosynthesis in aquatic plants. Maybe Tom would like to elaborate...

And thinking about it, I wonder if Anubias has stomata on its submerged
leaves? That might explain a few things. Tomorrow I will have a look under
the scope...


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