Salvinia leaves. III. Morphogenesis of the submerged leaf

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Abstract
Submerged leaves in Salvinia arise as single cells from the shoot apex. Shortly after initiation the primordium appears as a conical group of cells with two areas of lateral expansion toward the base. These two lateral regions plus the abaxial surface between them are the sites of initial leaflet production in the primordium. Following the appearance of the first set of seven leaflets, an additional set of leaflets is initiated and they become apparent as the submerged leaf emerges from the terminal bud. The growth of the submerged leaf is protracted compared with the floating leaves of the same set. Compared with the precisely located abaxial meristems found in Salvinia floating leaves, no discrete, regularly positioned meristems occur in submerged leaves. Thus, developmentally these compound leaves have little in common with the floating leaves or with other highly divided leptosporangiate fern leaves. The development of the floating and submerged Salvinia leaves is discussed with respect to leaf development patterns of widespread distribution in higher plants.