Shoot organization in the seagrass Halophila (Hydrocharitaceae)

The seagrass Halophila is characterized by a constant organization of its horizontal axes (rhizomes) and variation in the organization of differentiated lateral axes (erect shoots) that have diagnostic value. The lateral axis is shown to have positional and organizational homology. Branching throughout is precocious and always occurs above the insertion of the youngest appendage as a terminal bifurcation. It may be described as monopodial because the lateral axis is distinct from the parent (rhizome) axis in producing foliage leaves. Subdivision of the genus is based on the organization of the erect shoot and position of flowers, as indicated in a synoptic key. Terminal buds of the rhizome are distinguished by the large number of appendages that may be enclosed in the enveloping pair of scales, with up to eight visible branch orders. Despite this high degree of preformation and apparent continuous growth, there is some overall control of meristem development such that only a minority of apices are activat...