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Sinningia leaves are mostly pretty conventional. Like gesneriad leaves in general, they are not subdivided like Monstera leaves, or compound like palm leaves. There are no species where a cotyledon matures into a unifoliate plant (as in many Streptocarpus species), or where the petiole encloses a peduncle (as in a few Chirita species).
There are some differences among sinningia leaves, which can be of assistance in identifying species.
This fancy word (see the etymology) means the arrangement of leaves on the stem. Most Western-Hemisphere gesneriads have opposite leaves (that is, two per node), with the genus Gesneria being the main exception.
Sinningias either have two or three leaves per node. Two is the most common number. Even species which normally have three leaves per node have just two per node in juvenile plants. The pattern with leaves in whorls of three does not appear to be obligate in any species -- growing conditions can cause a reversion to two leaves per node on some stems.
The species which exhibit whorled leaves at least some of the time are
Most of these species are in the Dircaea clade, but it is unlikely that this is very significant.
A few species have unusual phyllotaxis.
Most sinningia leaves are plain green on top, but some species have more or less red on the undersides. The amount of red seems to depend a lot on growing conditions. As a general rule, the brighter the light, the less red. If grown in low light, Sinningia sp. "Black Hill" has leaves which are solid, intense maroon on the underside, but if grown in partial sun, the leaves may have only a tinge of red.
Species with red or purple leaf undersides under at least some conditions are
This is almost certainly not a complete list, since I do not have leaves of all the species available to me at the time this is being written.
In general, paliavanas and vanhoutteas do not have red on the leafbacks, but here is a cautionary example to warn against generalizing too quickly. My outdoor Vanhouttea pendula has ordinary green leaves, but a cutting I rooted under lights has a strong reddish flush extending outward from the ribs.
All sinningia leaves are hairy to some extent, but some are strikingly so. The best-known example is, of course, S. leucotricha, but it is not the only species for which leaf hairiness is an attractive feature. Species with conspicuously hairy leaves include:
Presumably, S. villosa has hairy leaves (the species name means "shaggy"), but I have never seen the plant, so I can't say for sure.
The most attractive feature of S. mauroana is the fine hairs which give the leaves a silvery sheen.
See a picture of a Sinningia hatschbachii leaf.
Some species of Sinningia have sticky leaves. I do not know the purpose of the stickiness; perhaps it's a defense against pests. Species with sticky leaves include:
It's interesting to note a taxonomic connection here; all except the paliavana are in the Corytholoma clade.
When a leaf (or other structure, such as a flower) has passed its usefulness, a plant will normally jettison it by creating a stratum of cells called an abscission (see the etymology) layer. This blocks transport of water and nutrients to the leaf, which then withers and dries. In most plants, such as a maple tree in autumn, the leaf eventually separates from the stem, cleaving along the abscission layer. In many sinningias, however, this does not happen, even after the leaf is thoroughly dead. The dead leaf and its stalk cling stubbornly to the stem. In sticky-leaved species like S. amambayensis, this can create a real grooming problem.
What the benefit to the plant is, I do not know. At certain times, it seems like a definite detriment, as when a grower tries to pull the leaf off, expecting it to pop neatly from the stem, only to have it peel a nice strip from the outer stem surface.