Members of the higher plants (the Division Tracheophyta
) have roots, stems, and leaves, and may have cones (Class Coniferopsida) or flowers (Class Angiospermopsida).
Apparent restriction of CAM to the Tracheophyta may be explained in part by the greater carbon allocation to cell wall material in these macrophytes, resulting in C acquisition being a more rate-limiting step than N, P, or Fe acquisition (Raven & Spicer, 1995).
Being restricted to the Tracheophyta means that CAM is found only in secondarily aquatic plants.
Despite this apparently very early origin for CAM, its widespread and highly disjunct phylogenetic distribution leads to the inescapable conclusion that, within the Tracheophyta, it is not a homologous trait (Luttge, 1987; Monson, 1989; Ehleringer & Monson, 1993).