How do Orchid roots actually work?

[King_of_orchid_growing:)]

Originally Posted by Fernando

But the question is still there: how do they actually work? Let me explain: where does the active/passive transport of nutrients an water happen?

The short answer is:

1. Diffusion.

2. Osmosis.

3. Capillary action.

Originally Posted by Fernando

If the velamen harvests water (including the dissolved nutrients), even for a short time until the entire liquids has been absorbed by the/into the vascular bundle, a overfeeding would be very likely to occur...

The velamen is inefficient with "harvesting water". It deflects a large amount of streaming water. It seems to only allow fine mists or a limited amount of small water droplets through.

It functions better as a form of water loss prevention for the cortical cells (i.e. cells in the cortex of the roots). This is one of the major functions of this thickened layer of epidermal cells.

The other function of velamen (or the function of any epidermal layer) is to protect soft tissues from physical damage. In this case, the soft tissues that the velamen is protecting is the root's cortex.

Originally Posted by Fernando

But if the velamen does not act a a sponge but acts as a barrier, than it would make sense to flush water regularly to clean substrate AND roots from excess of salts.

The velamen is spongy in texture, it doesn't necessarily act like a sponge.

Excess salts can build up on the roots as a result of the velamen acting primarily as a "barrier", rather than functioning as a highly efficient sponge. Like I said, it doesn't seem to allow certain things through in large quantities.

[Fernando]

Yes and no...
The velamen does get wet and even if it dries up quickly, it retains more water than a bare root. The efficiency may be low, but always when water is going through capillar sistems, there is a salt built up. I'll show a pic of my little Encyclia seedlings growing in lava ashes; there are a few twigs of dead moss, where the little leafs have concentrated salts that "bloom" at the tips of it - the same way the dead velamen cells would concentrate salts if they could freely rush through!
Besides, there is also an active taking up of nutrients in root cells. Where does this take place in orchid roots? Over the whole length of the root or just in the green tips - where there is no velamen?

I think orchid roots without growing tips are still active - in fact they can regrow sometimes laterally or even at the old tip - this would mean, they can take up water over the whole length even if not growing any more - and this is different to other plant, isn't it?

Fer