That little word PLEASE wouldn't go amiss Cothey. The answer is Concave, like a saucer waiting for the tea cup. Do not plant deeply, a good half of the tuber of the tuber needs to be above the compost level. The roots need moisture but not the upper part of the tuber, Jenny namaste.
well I wouldn't recommend planting plug plants or whole plants the wrong way up , but tubers, it doesn't really matter in my experience, but I'd go with Jenny's answer, but they'll all reach for the sky The best thing about Begonia tubers is that they stand a chance of coming back after winter if left in the ground.
Yep concave up I say, half buried, once growing well top up and cover the tuber. Well that's what I do, anyway.
Sorry, are we talking tubers or corms here ? I think of tubers as underground thicknesses - like fingers . And corms as the dark hard upturned mushroom like things. Must be getting tired or old and confused.