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[APD] RE: Roots

>Probaby you all know the phenomenon - roots emerging from the stems.
>So here goes the hypothesis - the plant doesn't have enough nutrients so
it sends those roots in order to optimize nutrient sucking. If you add a
liquid fert then it won't >happend.
>What do you say? True?

I'd argue not true.
The plant has little issue getting what it whats/needs from the water
column through the leaves. There are a few studies that removed the roots
and measured growth and found that the plants grew the same if there was
enough nutrients in the water column. 
Some less aquatic plants do need some root supply of iron for better growth
Issue number two, those aerial roots possess no root hairs where uptake
occurs on the plant's roots. So having them provides no advantage for
nutrient uptake vs the leaves.

So why do plants send roots out?
This is unresolved but a few possible reasons for this:

 Often the plants grow in shallow water, eg Ludwigia repens will send
aerial roots out and grow in the shallow water but not have enough support
to rise above the water much. Like many plants that have runners/stolons
and vegetative propagation, this is similar. The plants will creep along in
water and once the roots finally hit pay dirt they attach and form root
hairs in the soil and spread out. These can be emergent plants as well as
submersed plants, most places have high variations in the water levels
throughout the seasons. Florida has only about 3 feet or so vs 30-50ft in
SEA, SA, Africa and most places in the tropics that have monsoonal weather
This water level variation is ripe for fragmentation dispersal adaptation
and many plants are amphibious. But how can a plant live under 30ft of
turbid tannins filled water with no light? Fragmentation and the ability
for form roots at each node would be helpful. If you reduce the light down
below the plant's minimum level(LCP), many stem plants will melt in regions
and fragment. Often if you leave plants in a bucket too long this will

These aerial roots, if fragmented from the main plant, can send roots
quickly into the soil downstream.
Many aquatic plants melt and fragment when conditions are declining so that
the fragments might find better conditions.

 Often a plant will be pushed down with the current and if one part becomes
dislodged, the other will hopefully hold it down and the combined strength
of several sets of roots at each node will hold the plant down better.
These types of plants also fragment and disperse this way as well so it
helps to have the ability to form roots at each node.  The stems of many
aquatic plants are not very strong, they break easily. They don't have much
support due to their habitat and water bouyancy. 

Sending more roots down will hopefully supply the plant with more CO2 and
nutrients sure, but you can add nutrients and CO2 and the plant will still
do this. If it based on nutrients, perhaps the plant is "fooled" by adding
CO2/nutrients? I'm not too certain it's a vestigal trait. I'd suggest other
reasons. Without CO2 I really don't see many plants do this nearly as much.
So perhaps growth rates play a role also. 

But it's unresolved yet why they do this, it's some sort adaptation to the
environment but I think it's similar to stolons, a form or fragmentation
rather than nutrient uptake. There are other possible reasons for this than
I've given, but nutrient uptake seems unlikely for a reason.

Tom Barr 

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