The Structure and Relationships of the Potamogetonaceae and Allied Families

Abstract

1. The course of the leaf traces in Potamogeton is not typically monocotyledonous, for in the more robust species the three traces run down through one internode at the periphery of the central cylinder before entering the medulla, where they descend through a second internode, finally fusing with cauline strands. 2. At the points of fusion the bundles are found to be amphivasal, while they are collateral throughout their internodal course. 3. The phloem of the bundles at the base of a branch shows a marked swelling. 4. The cortical bundles present in some species of Potamogeton are not regarded as a primitive feature, since they do not occur in the fertile part of the floral axis and occur only sparingly if at all in the creeping stem. They enter the stem as small strands from the stipules and petioles and are not as a rule connected with the principal leaf traces. In Zostera and Cymodocea, however, there is a system of cortical bundles which are connected with all of the principal leaf traces except the median one. 5. The circle of collateral bundles found in the floral axis in Potamogeton and Triglochin is regarded as a relic of the ancestral dicotyledonous condition of the vascular system. 6. The evidence afforded by the vascular and floral structures indicates that Potamogeton is the most primitive genus of Potamogetonaceae. It shows more clearly than the other genera features characteristic of terrestrial life, though some of its species are apparently reduced forms. 7. Species of Potamogeton having both floating and submersed leaves and a central cylinder in which the bundles run separated from one another are regarded as the most primitive members of the genus. P. pulcher fulfils these requirements. 8. The other genera of Potamogetonaceae show further stages of reduction in accordance with their totally submersed habit. Their relationships are in some cases shown by the vascular structures. 9. In Naiadaceae the reduction is carried to such an extreme of simplicity that the affinities of the family are obscure. 10. Aponogetonaceae and Juncaginaceae are more typically monocotyledonous in their structure, and do not seem immediately related to Potamogetonaceae.