Cool question...

The folks at Humboldt state have come through with a nice discussion of gas exchange.

Basically, your lavendar and roses do most of their gas exchange through their roots. That's why a loved but over-watered houseplant drowns to death.

From what I read, the water plants do much of their gas exchange through their leaves.

Here is a link that might be useful: Humboldt State explains gas exchange in shore, near shore and aquatic plants

Roots need oxygen for respiration (aerobic respiration). In the beggining of flooding when oxygen is present but only at low levels, the roots start producing ethylene. Ethylene will help your plant cope with these conditions but it is a senescence hormone at the same time. So your plant will start wilting and yellowing. If flooding conditions persist, whatever oxygen was left in the water has now totally disappeared, and in this case your roots literally ferment (= anaerobic respiration). Oxygen is used as a terminal electron acceptor in mitochondria, and in its absence the whole respiratory chain is blocked. At this time all ATP is used in the roots and in order to create more ATP, the cell roots will start breaking down starch. The whole process of starch breakdown , without oxygen, leads to the production of certain molecules that are detrimental to the cell. So at this point the plant dies.
HTH

Josh : water rooted plants grow a special tissue (aerenchyma) in the roots that enables them to aerate themselves. That's why many ppl arent lucky when they pot up water rooted plants. The roots need to adjust themselves in soil but they don't have time.. so the plant ends up dying.

Josh
I have a varigated form of Ficus benjanina (tropical tree) That had just had its first root pruning so it could be grown as a bonsai. I placed the plant in a bucket of water and was gone for 6 weeks due to an emergency. Whem I returned I fully expected to see the tree drowned lol
Another problem arose and I was gone for 3 more weeks.When I returned it had started to put out new growth!!
Now I was curious as to how long it could take the abuse so I left it in the bucket. That was in 1987!! That Ficus is still thriving though I've had to change the bucket several times due to leaks.
Plants truly are amazing!!!
Gary

So, that explaines my Easter Cactus. It's been growing in a glass of water for two years.

Do water plants have aerenchyma in their roots, or do they depend strictly on the air passages mentioned in Bradley's link? Thanks Bradley, that's an interesting site, I'm going to go back and explore more of it later.

Thanks, Nazanine. I find it so interesting that the plant can adapt itself to water. And now I understand a lttle better why I often have better luck just taking a new cutting than trying to make those water-grown roots adapt to soil.

Gary, Ficus roots scare me...I'd be very very careful near that plant. It's a real survivor! lol

Peter...that's so strange that even a succulent can adapt to long-term water culture.

I really like this forum...I've learned a new bit every day. josh

Amazing scientific info and anecdotal comparisons.
Plants are tough.
From my own experiences, I have discovered that more than anything else that can be done to offset the shock of moving cuttings from water to soil or water rooting cuttings cut from a soil growing plant is the temperature of the water and the ambient air temperature. Succcess usually occurs when I pot up water rooted cuttings and grow them in precisely the same place that they were rooting in for a few weeks. Likewise, if I take cuttings from a soil growing plant and place them in water at the same spot where the parent plant is growing for a few weeks before moving them success is most likely. Microclimates that we mammals can easily adjust to may be more shocking to plants.
Dave

Hi
Another little trick I've learned is to bubble air into the rooting water.Have at least double the success of standing water. There also seems to be a relationship between the humidity and light For fun I tried bubbling CO2 instead of air in the water..Mixed results. One of the best
is to stick the plant through a piece of cork and float it in an aquarium.Have at least 90 percent success This I use mostly to get them to grow without media or epiphyticly.
Ay least 70 percent humidity seems to be necessary to get them to continue to grow.It also must be done slowly.
Gary

Dave, You reminded me of 40 years ago and a very old lady gardener (no disrespect intended...I'm one now!) who insisted that you had to root a rose (and possibly other shrubs?) right by the mother plant. She took rose cuttings and placed them in ground near base of rosebush and covered with a mayonnaise jar. Must have been successful as she was known for her roses and had a nice lttle homebased income selling them in tincans. I don't remember what time of year she took the cuttings. I was a very new gardener just into houseplants mostly but agog with amazement at learning what plants would do (still am). josh

Another thought on rooting roses near base of mother plant: it may have started as a way to identify the new cutting later after rooting and before blooming. Of course as soon as cutting was potted up confusion could and no doubt did occur. People didn't often use plant labels in small nurseries in those days. Buyers often ended up with a red when they thought they had bought pink.

The Azalea growers of those days must have been particularly bad at keeping colors separated and marked...! All the buyers couldn't have wanted one of each color, lol josh

Jo,
....."who insisted that you had to root a rose (and possibly other shrubs?) right by the mother plant. She took rose cuttings and placed them in ground near base of rosebush and covered with a mayonnaise jar".....

Most interesting. Part of this may support my own experience that if a cutting, no matter how it is rooted, is maintained in the same micro climate as the parent plant, greater success is probable. The mayonnaise jar part contradicts this theory as in essence, a green house has been set up.

Dave

Dave, On second thought it must have been in late fall and the jar may have acted like a coldframe? I think in GA summer a rose cutting would have fried under glass.

I've seldom rooted any plants outside other than sticking cuttings of tropicals (mostly) into same pot, but it's the same principle. You have the same soil and moisture level, air temp and sun or shade as on mother plant. I'd never really thought much about it but it makes sense! But then my plants are mostly those which are easy to propagate by anyone. josh