Bumping this query up. Serious, inquiring minds would really like to know.

How they differ morphologically is what I have so far failed to find.

I've heard that plants in hydroculture have water roots, is that true?

Water roots seem to be more straight and more brittle in my experience.

I'm Lazy--What can you root in plain old water??? - ...''but the physiology of roots that grow in an aquatic environment is such that it allows diffusion of oxygen from above the waterline to fuel root metabolism. This requires a much different type of root tissue that the parenchyma found in roots that grow in well-aerated solid media/soil. ...''

Microsoft Word - 24videmsek.doc - 24videmsek.pdf - ABSTRACT
The formation of root aerenchyma, the prominent air spaces in t
he root cortex which are
normally induced by waterlogging,
has an important role in prov
iding an internal pathway for
oxygen transport between roots and the aerial environment. Alon
g this pathway, O
2
is
supplied to the roots and rhizosphere, while CO
2
, ethylene, etc. move from the soil to the
shoot and the atmosphere. The aerenc
hyma are formed either as p
art of normal
development, or in response to stress (e.g. hypoxia, nutrient d
eficiency), by two known
mechanisms: schizogeny and lysigeny
.

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Beyond this point it begins to get complicated.

only thing I've observed is that water roots seam to be finer than roots grown in soil.

What I keep reading is that in hydroculture with leca beads all the roots are water roots. I can understand that roots that grow down into the water reservoir develop into water roots, but why do the roots above? How is that different to a gritty mix which is also very aerated?

Not all of the roots grown in LECA (semi-hydro) are water roots, but any that grow into the reservoir will be.

The roots that adhere to the growing media will be your regular "terrestrial" roots.

Any part of the terrestrial root system that stays in the water reservoir will take on characteristics of water roots (being finer and having more root hairs), but the root system parts that don't stay waterlogged will be of terrestrial nature.

This essentially means that even the terrestrial roots will become like water roots, if long enough.

Plants grown straight hydroponically (versus semi-hydro) will only have water roots.