In spite of the common misconception of bryophytes as nonvascular plants, many detailed studies have demonstrated conducting strands in mosses and liverworts. The ultrastructure of the water-conducting cells in Metzgeriidean liverworts, such as Symphyogyna and Pallavicinia, have been well described by Smith (1966), Frey et al. (1996), and Ligrone and Duckett (1996). Central strands in different taxa of Metzgeriidean liverworts, in fact, show different levels of cell differentiation. The purpose of this investigation is to describe the structure of central strand cells in several simple thalloid liverworts, including Jensenia, Hattorianthus, Cavicularia, and Calycularia. Studies with optical and electron microscopy show that both the rhizome and the thallus midrib of Jensenia possess a central strand which is composed of thick walled, small diameter cells. SEM study shows that these cells possess numerous, oblong shaped pits on the inner surface of the wall and are connected by oblique end walls. These central strand cells of Jensenia are, in fact, structurally very similar to those in Symphyogyna. The massive midrib of Hattorianthus possesses two central strands. The cells in the strand are smaller in diameter than surrounding cells and have brownish coloration. SEM study shows that end walls of some specialized parenchyma cells in Hattorianthus possess numerous small pits which seem to be of plasmodesmatal origin. The genus Cavicularia possesses three differentiated central strands in the midrib. The cells in the strand are smaller in diameter, but do not possess any wall thickenings. The cells of the thick, conspicuous midrib of Calycularia are filled with many starch grains, but there is no differentiated central strand. Continuing studies, using TEM and histochemistry, will focus on the internal structure and function of these cells.

Key words: central strand anatomy, liverworts, pits, SEM