re: Why do roots exist?

>  From: Jessica Dion <jdion at mole_uvm.edu>
>  According to our speaker thisevening,
>  aquatic plants absorb most nutrients from their foliage and have no need
>  for a major root system with root hairs, etc. the wat terrestrial plants
>  do.  

I would tend to disagree with the speaker in the claim that aquatic 
plants have no need for root hairs (more on this later).

>  3. The success of liquid supplements would seem to suggest that the above
>  claim is true - that plants can absorb nutrients through their foliage,
>  and don't depend on roots for that - that perhaps roots are chiefly there
>  to help anchor the plant.  How does this claim rest with you all?  If this
>  is a reasonable statement, why is substrate that important - why bother
>  with laterite/soil etc. when you can just grow plants in gravel and
>  supplement the water with nutrients?  I'm just curious, but these
>  statements just don't seem to mesh with everything I've learned until now. 
>  Other members of my club swallowed these claims quietly though, which is
>  why I'm bringing ithem up here. Thanks for any ideas you might have on
>  this.. 

I have anacharis, H. difformis, H. polysperma, Several crypts, 
Lil. brasiliensis, N. stellata, and several other species in my tanks.  I've
found that plants in a fine substrate (possibly a fine substrate with a clay 
like laterite, vermiculite, etc) tend to produce root hairs.  Even the 
anacharis roots produce extremely fine root hairs that tend to pull up some
of my vermiculite when I remove the anacharis from the tank.

In addition, I fertilize my tank almost solely through the substrate (through
a nutrient injection manifold that I installed during setup).  This may affect 
the formation of root hairs significantly.  Fertilizing through the substrate 
doesn't seem to be a factor in preventing root growth in the water column,
though.  My Hygrophila, Anacharis, Alternathera, and Hydrocotyle are quite
good at putting out roots in the water column.  The only stem plant that seems
to keep its roots to itself in my tank is the Rotala rotundifolia.

If I were to speculate, I'd say that root material provides a significantly 
greater surface area/plant mass ratio than leaf material.  I'd speculate
further that the high surface area assists significantly in nutrient uptake,
and might help the plants to compete against algaes as well (algaes have a
high surface area/mass ratio).  I'd continue to guess that if the substrate
doesn't provide a rich enough source of nutrients for the plants (I probably 
run short on some nutrients right now), the plants will attempt to get them
from the water column.

This speculation leads to the following question:  Why don't plants form root 
hairs in the water column?  I'm gonna keep guessing, but I think they don't
do it "because" the root hairs wouldn't be protected from consumption by 
other organisms.  I know my platies like to graze on H. difformis roots.  If
I pull up a bit of H. difformis and leave it floating, the platies will 
clean it of root hairs within several days.

David W. Webb
Enterprise Computing Provisioning
Texas Instruments Inc. Dallas, TX USA
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