The Evolution of Achene Morphology in Ceratophyllum (Ceratophyllaceae), I. Fruit-Spine Variation and Relationships of C. demersum, C. submersum, and C. apiculatum

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Abstract
Variational studies of Ceratophyllum apiculatum, C. demersum, and C. submersum demonstrate that C. demersum and C. submersum are separable by multivariate statistical analyses, whereas C. demersum and C. apiculatum are not. In fruits of C. demersum, stylar and basal spine lengths are highly correlated and related linearly. A comparison of interspecific spine-length variation with variation observed within a clonal population (identified by enzyme electrophoresis) indicated that a large component of spine-length variation in the species is genetic. In C. demersum, the frequency distribution of basal spine lengths is bi-modal as a result of spine abscission. These results provide evidence that C. apiculatum fruit phenotypes are simply variational phases of C. demersum. The data are useful for inferring possible evolutionary relationships in the genus.