No idea why the pic wont show - next try:

Here is a link that might be useful: Begonia

It is a rex begonia but too young of a leaf to tell. Have you got any pictures of the full plant? If you are trying to propagate it I would push the stem all the way into the soil.

Oh, the leaf is fully grown, it is not a rex but a small rhizomatous one - the adult leaves on the big plant were no more than 6 or 7 cm. I just got this leaf s cannot post a photo of a frown plant (yet).
I rooted the leaf in water, and as small rhizomes are forming at the base of the petiole I did not want to bury them too deep.
Still I'd be happy for any advice!

p.s. are there rex begonias with a velvety leaf surface? I have never seen such but it would be a great thing!

Yes there are many rexes with velvet like leaves and many dwarf or miniature rexes too. Mini-Merry for example.

As for rooting I see there are signs of new growth at the leaf/petiole union which are usually very good signs of rex heritage.

Wow, thanks, it seems like I still have to learn a lot about these Begonias - I just knew the old-fashioned large Begonia rex but will see that that changes.
Are those red things signs of new growth? And will there be growing tips at that place?
Actually, I can't remember if they have been there or formed, but the other end of the petiole has a lot of roots already and tiny rhizomes forming.

Yes the red things are signs of new growth.

Rexes are not the only leaf to do this but most other rhizo begonias don't exhibit the same behavior.

Here is an exception:

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This is a rex that I don't remember the name of but it was rooted in water:

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I think these two leaves are from Begonia 'Peace'

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Wow, amazing, thanks for sharing!
At the moment I potted the leaf in a way that most of the Petiole is not in the soil - will die adventive plantlets still sprout out of the leaf or do they need contact with the soil? I wonder if I should plant the whole thing vertically, so that both plantlets at the lower end of the petiole and the ones at the leaf base are in contact with soil though not buried?
I wonder why it took me so long to start again with Begonias....they are such amazing plants!

Lots of questions there. Some begonia leaves will root in air if the humidity is high enough yet others will rot or dry up. As for pushing the petiole all the way in the soil and keep the plantlets alive at the cut - well that depends on how long the petiole is and what the media is composed of. If it was straight perlite I'd say you wouldn't have a problem. If it is heavy soil and the petiole is more than an inch long then you might smother the growth you already have.

What I would do now is let the plantlets get a little larger, then cut the petiole close to the soil and trim it to about an inch long, then push the petiole in some more quality potting mix or perlite, and let the plantlets at the petiole union get rooted. Some growers can use the same leaf many times over (depends on how many plants you really want).

Thank you for your good advice!
Meanwhile, I dug out most of the petiole and arranged it in a way that the petiole is almost vertical on the soil surface and the leaf blade is close to the soil - maybe that way I can encourage growth of both growing centers. Potting it into perlite is an Idea I should have had myself - maybe I should repot it into that medium?
I'll keep you updated and want to thank you for the great advice!

Michael

It's Camelot.