I can stick a leaf from an african violet in some water, and eventually it’ll root and start a new plant (if it doesn’t rot first). But with other plants, I need a branch, not just a leaf. Why is this?
Does this have anything to do with the fact that african violet leaves grow directly out of the plant (which doesn’t branch out), so the stem of the leaf is actually a sort of branch? And isn’t this an example of cloning?
Yes, it is cloning (it’s even actually called that in horticultural circles); different plants have their tissues arranged in different ways; African violets happen to have root-forming tissue in the leaf stems, moreover, the leaves are fleshy and so they can act as a reserve for water while the new plant is getting the root system set up.
Other plants either don’t have the root-forming tissue in the leaf stems (so you have to tear off a ‘heel’ from the stem, or use a larger piece; some plants have the tissue there, but don’t react to injury by growing roots, at least not very quickly, or they don’t have the food and water reserves to sustain life while the root system is forming (so you need a bigger piece of plant).
You can speed up the formation of roots with “rooting hormone” from any gardening store. The trick is to use as little as possible; it makes the cutting start roots, but then inhibits their growth. Dip the cutting and then flick it hard with your finger to knock off all the powder you can.