Roots in medium vs outside of pot: are they all air roots?

[karrolhk]

There seem to be different types of roots in an orchid:
1. roots in traditional medium (bark and moss)
2. roots in water culture medium (just water)
3. roots that grow outside the pot

Are they the same: i.e. all air roots? Do #1 and #2 have velamen like the air roots? I have read that #3 have velamen but they won't be able to absorb water when put in the medium.

Or, are they different? How are they different? because #2 is adapted to growing in water, what makes #2 different from #1 to be adapted to growing in water? If they are different, do they have different names?

Thanks

[Roberta]

For epiphytic orchids (most of what we grow), the roots in the medium and the roots that grow outside the pot are the same - they really want "humid air" rather than "wet". Roots tend to adjust to their environment, so that if you want to change (like from bark to semi-hydro, or just old bark to new bark) the ideal time is when new roots are just starting to emerge - they will grow into the medium and adjust as they grow. Roots that grow outside the pot totally in the air are the same, but they're more adapted to the air... if you are getting a lot of them and haven't potted the plant in awhile (or ever) the roots in the medium may be suffocating from a wet, airless enviromment and the flush of air roots are the plant's attempt to get the air that they crave. Sructurally they're the same - the velamin is like a sponge, holding water reserves. Growing in water is unnatural... in semi-hydro, roots may grow down into the reservoir and adjust to being in water, but they will still have a lot of surface in the air. People claim success growing orchids in water, but I wonder for how long... one should not make a claim about the efficacy unless the plant has actually survive that way for a couple of years... orchids don't do anything fast, including die...

[camille1585]

Roots are roots whether they are in a pot, air, or water and will have the same cell structure (going back to your question, all have a velamen). The difference is in the properties the root develops in a specific environment. The velamen of roots which develop in air will usually be thicker (to hold more water), with a harder external layer to provide added resistance to dessication. How wet or dry the environment is will change of the properties of the cells as they develop (number of cell layers in velamen, the structure of the cell walls…). Studies have shown that when the velamen comes in contact with bark (or any other substrate) genes which control velamen development switch off early and root hairs develop instead of additional layers of velamen. So with a thinner layer of velamen and all those root airs, these pot grown roots would quickly dry out if exposed to the open air. Roots growing in water will be missing even more of the hardened outer layer of velamen.

As to why some roots grow into the air instead of down into the pot, it is often because they are either searching for a better environment (ex, the substrate is broken down and suffocating the existing roots), looking for moisture (pot is underwatered) or the plant is looking to attach to something for extra stability. Air roots which find something to latch on to will often flatten out as the grow on the support they found, to increase the surface area they attach to.

[karrolhk]

Thank you so much Roberta and Camille for the wealth of information.

Camille mentioned about roots growing root hairs instead of velamen when in contact with bark. Why do they grow more root hairs when in bark?

Also, if the roots growing outside the pot are the same as those inside the pot, why does this academic article on orchids say:

“Aerial roots of epiphytic orchids cannot absorb water from the soil but supply the plant by collecting atmospheric water together with dissolved nutrients.”

The Velamen Radicum of Orchids: A Special Porous Structure for Water Absorption and Gas Exchange | SpringerLink

[estación seca]

Aerial roots can't absorb water from the soil because they're not in contact with the soil. The abstract explains how they function.

[twinkie]

Hmmmm...I've been wondering about roots too.
I repotted most everyone about a year ago so the medium shouldn't be breaking down, but I have noticed a surge in 'air' roots and only calling them air since they're not going down into the medium.
Some of these are so long I've had to coil them around up top the medium and try to encourage them to go 'down', not sure if that's smart to do.
I've also got new roots coming out much higher up on a plant between leaf sets, which I find strange. That mini phal has 4 roots coming out in successive levels between leaves with normal two sets of four leaves below with zero roots coming out between them. The plant feels secure and rooted - no wiggle - so not sure why this particular plant is doing that. None of the other larger phals are doing this 'between the leaf sets' of roots shooting out.
One larger phal is sending a root straight out into the air parallel to the ground for about 8" - which I find odd lol. Another is actually sending two roots straight UP, and they're def not bloom stalks.
Most all pots do have roots coming out of their lower side holes and tons coiled around up top , like lots of roots everywhere so when it comes time to repot I'm always worried about trying to force these upper roots down into medium.
I liked the explanation about the harder vellum which explains the texture and appearance of some of my surface roots. I was thinking it was somehow a 'burn' from fertilizer but I do extremely weak so wasn't sure.
Is a good rule of thumb to repot (even if the medium is not broken down) when too many roots are popping out pot sideholes and waving around up top?
Most all my plants have 4 to 5 sets of leaves and really robust root systems to where the roots are almost seeming to push the plant up out of the pot over time from its original planting depth. ]Well this is all over the place but I'm positive the orchid wizards on here are gonna deliver wisdom! Thank you!

[karrolhk]

I'm a little confused.. in another scientific study, it says the velamen absorbs water but it doesn't go into the cortex of the roots. And so the function of velamen is not really to absorb water but just to provide mechnical support:

"The intact mature velamen and exodermis of the aerial roots of certain orchids seem to be nearly impermeable to water and certain solutes. Though the velamen may absorb both, little of either seems to penetrate into the living cortex within. Radioactive phosphorus could not be detected in leaves of orchids even after aerial roots had been immersed in a solution containing it for as long as 96 hours.

... The principal roles of the velamen in free aerial roots are mechanical protection and the prevention of excess loss of water from the cortex."

Source: JSTOR: Access Check

[SouthPark]

karrol ----- some other mechanisms likely are involved as well ---- regarding roots that is. Depending on what environment or conditions that newly 'forming/growing' portions of roots are exposed to ----- some internal biological/physical/chemical mechanisms will be involved --that configures the roots (including the various sorts of cells and internal systems) in a way - for the benefit of the plant's survival.

Some of those configurations could be 'permanent' ----- as in configured permanently during the growth period. So - depending on circumstances - the roots that are 'adapted' for particular conditions might not function as well (or at all) under significantly different conditions.

Eg. roots may have adapted or have their cells configured for watery and lower-oxygen-content media conditions ----- that might work ok in air too ----- maybe. But regular roots transferred to water might not work at all.

[aliceinwl]

I’ve potted lots of Phalaenopsis air roots. There where a couple of rehab plants from a co-worker that had nothing but air roots. The roots all survived repotting and changed morphologically to be indistinguishable from roots that naturally grow into media. If roots are healthy, they can adapt to changed circumstances. Unhealthy roots tend to die.

As a matter of preference, I repot my Phalaenopsis when the roots get too crazy because I like to reuse the pots (I don’t want to have to cut a significant portion of the roots in order to extract a plant) and, I don’t have space for masses of air roots. I bury the air roots when I repot. Under my husbandry, even plants that produce tons of air roots just about always have a pot full of healthy roots too so I’m continuously upsizing pots.

[estación seca]

What alicinwl said is important to remember - orchids of all kinds growing well in good conditions make LOTS of roots, whether aerial roots or in the pot. If your plants don't have LOTS of roots, look into light, humidity, temperature, watering and air at the roots.