There are three successive phases to foliolar development in Parkinsonia aculeata: (i) an initial phase during which primordium formation occurs; (ii) a morphogenic phase, with foliole formation; (iii) a histogenic and differentiation phase, with foliole development. Foliolar shape is determined during the basic morphogenic phase; a longer phase in which a zoning effect analogous to that seen in branches has been observed: (i) a zone of distal organ formation where folioles appear in constant succession, in a specific location, and in close proximity of each other along each border (these are the three criteria which define intrafoliolar generation centres); (ii) a histogenic median zone; and if the foliole is old enough (iii) a subproximal differentiation zone; (iv) an adult proximal zone.