The Taxonomic Significance of Plumule Morphology in Ceratophyllum (Ceratophyllaceae)

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Abstract
The plumule morphology of the species of Ceratophyllum was studied for the purposes of: 1) determining the extent of variability in forking of the first epicotyl leaves, and 2) evaluating plumule features as taxonomic characters by assessing interspecific and intraspecific (inter- and intrapopulational) variation. Embryos from specimens collected throughout the ranges of eight of the species (Ceratophyllum australe, C. demersum, C. llerenae, C. muricatum, C. oryzetorum, C. platyacanthum, C. submersum, and C. tanaiticum) exhibited no variation in morphology of the first epicotyl leaves. In all instances the leaves were simple. In addition, no intrapopulational variation in the character was detected in C. demersum, C. llerenae, or C. submersum (rarity of material prohibited study of intrapopulational variation in the other five species). In contrast, embryos from specimens collected throughout the range of the ninth species, C. echinatum, possessed forked leaves at all nodes of the plumule. In C. echinatum, interpopulational variability was observed in the number of segments produced by the forked plumule leaves at the first node. The segment number is not correlated geographically, and most combinations of segment number can be observed within a single population. The unusual plumule morphology of C. echinatum is distinctive within the genus and has probably resulted from phylogenetic reduction of the lower plumule axis. These observations support recognition of C. echinatum as a distinct species.

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