Conductive tissue in Ceratophyllum demersum (Ceratophyllaceae)

Using scanning electron from microscopy (SEM) and light microscopy, we attempted to establish if xylem and phloem occur in Ceratophyllum stems; roots are lacking in the genus. In larger stems, a central core of elongate cells contains scattered sieve tube elements with simple sieve plates; the sieve tubes are associated with companion cells. Sieve tubes are not organized into phloem strands. The central core of main and branch stems contains elongate cells, some of which lack starch and tannins. These cells lack any evidence of the annular or helical thickenings of secondary wall material characteristic of primary xylem in other vascular plants, and therefore xylem cannot be claimed for Ceratophyllum. Absence of xylem in Ceratophyllum is interpreted as secondary, a loss related to adaptation to the submersed habit.