A Study of Stelar Ultrastructure in the Heterosporous Water Fern Marsilea quadrifolia L

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Sieve tube elements occur in the rhizomes and petioles of Marsilea quadrifolia. These are either thick walled with compound sieve plates in oblique end walls or thin walled with simple sieve plates in transverse end walls. Vessels are restricted to the metaxylem in the roots where the phloem contains sieve cells only. The sieve pores are invariably callose lined and as in other pteridophytes, excepting the Lycopsida, refractive spherules are ubiquitous in the sieve elements of Marsilea. The lumina of the protoxylem tracheary elements in the rhizomes and petioles are occluded by tyloses but probably remain functional in the roots. Pericycle cells backing on to the root protoxylem arms possess wall ingrowths. Transfer cells are however absent from the vascular tissue of the rhizomes and leaves. It is suggested that their presence in the root pericycle is related to the retrieval of ions from the xylem sap which may be particularly critical in water plants. The incidence of transfer cells in cryptogams appears to be far more sporadic than in angiosperms. The root endodermis of Marsilea possesses a casparian strip and abundant vacuolar tannin deposits. Plasmalemmasomes are numerous adjacent to the pericycle transfer cells.