Comparative shoot development and evolution in the Lemnaceae

Shoot development in three duckweed (Lemnaceae) species (Spirodela polyrhiza, Lemna minor, Wolffia borealis) was studied and compared. Duckweed shoots are extremely reduced, and no evidence of a shoot apical meristem was seen during development. Duckweed shoots generally consist of a single unit (a frond) that is interpreted as a developmental hybrid (of leaf and stem origin) and may best be described and conceptualized as a metameric unit tan internode and associated node with its appendages). In the pocket(s) of older fronds, successive buds arise from meristematic tissue at the base of the previous bud. This bud development appears homologous to supernumerary bud development (multiple buds at a node) that occurs in Pistia stratiotes. First-formed buds and pockets develop from tissue on the dorsal surface near the base of frond primordia. We suggest that the morphology of Lemnaceae plants can be understood as a result of the progressive simplification of shoots from Spirodela to Lemna to Wolffia, all of which have evolved from a Pistia-like shoot system. Understanding the shoot architecture of duckweeds has helped to clarify the growth potential of these prolific plants.