[APD] Re: Roots on Stem plants

["s mifsud" <labmdp at waldonet_net.mt>]

Tom said:
"So why do plants send roots out?
This is unresolved but a few possible reasons for this:
...
I'm not too certain it's a vestigal trait. I'd suggest other
reasons."


If we can classify angiosperm aquatic plants that we grow on an evolutionary
basis:
starting with the least aquatic:
1) (the otters)  the flat, round leaved types such as Ludwigia, Mentha,
Hydrocotyle - plants that can grow submerged, but normally grow and grow
better in marshy land conditions
2) (the seals)  plants with narrow/filamentous submerged leaves with aerial
flowers (and sometimes aerial/floating leaves) eg Cabomba, Ranunculus
3) (the dolphins) plants with only submerged narrow/filamentous leaves and
submerged flowers   eg Zanichella, Callatriche

It is quite clear to me that it is the first group of plants that produces
'aerial'/node roots most frequently in aquariums. Zanichella for example
will only produce roots at nodes but only when the stem comes in contact
with the soil. It 'knows' when it needs to root and when it does not.

The group (1) plants are accustomed to growing on wet land horizontally,
rooting frequently so as to cover as much ground as possible. Rooting and
rooting quickly is an important adaptation to this type of habitat. They
continue to do so even if submerged. I don't think that they have evolved
the ability to switch the rooting mechanism off when not required. Producing
unnecessary roots is a waste of energy that more evolved aquatics have
eliminated. So I do think that this lack of control can be thought of as
vestigal/primitive trait in 'aquatic' stem plants.

If the primary reason for 'aerial' roots was as a kind of anchor for
dispersal by fragmentation would'nt we see the trait also on the more
advanced aquatics, just as often?


Best Regards
Stephan






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