It has been proposed that one of the plants described in the St Francis painting may be ivy leaved toadflax, which Bellini describes tumbling down a craggy wall on the top right of the picture space into the saints rocky, contemplative environment. A delicate, trailing plant, toad flax is known for seeding into walled crevises, pavements, rocks and stoney places. It has smalll, ivy like leaf forms (which are edible) and small purple flowers in the spring with red stems.
Bellini describes the plants botanical form accurately here suggesting close knowledge and understanding of the plant, made in situ. The appearance of toad flax which I found here in a street in Castello, and in may other part of Venice, suggests that as with other plant details in the St Francis painting, botanical studies of such plants may have been made in Venice.
As with other plant species in the St Francis painting, Bellini's choice of species may be symbolic as well as particular to a Venetian environment. When in bloom, the flower-stalks bend towards the light, whilst once the flowers are finished, the seed-heads bend the other way, so that seeds are more likely to be shed into cracks in the supporting stones. If St Francis painting functions as a celebration of the smallest details in nature and a veneration of light, the choice of this plant amongst other more obviously symbolic species such as the laurel, is fitting.