Leaf Structure of our Mimosas and Acacias
by Bob Harms
The mimosa leaf (and that of many other legumes) is doubly divided (bipinnate), first into pairs of pinnae and in turn each pinna into pairs of leaflets. [Pinna, plural pinnae is from Latin pinna = penna 'feather', from which also our word 'pen.'] The picture below shows one leaf from each of our three mimosas. The number of pinna pairs are 7, 3, 2 respectively (each one is numbered). The number of leaflet pairs for the pinnae enclosed in yellow is 13, 5, 6 respectively.
By contrast the related Sophora affinis, Eve's Necklace (below), is compound pinnate - with leaflets alternate (not oppositely paired) and an odd leaflet at the end.
Plant Resource Center Home Page — Flora of Texas
— Mimosa