Psilotum

DIVISION PSILOTOPHYTA

This small division (one family with two genera) contains the simplest of all living vascular plants, Psilotum and Tmesipteris (Fig. 1; Table ). Psilotum is constructed very much like Rhynia, and until the mid-1970s Rhynia, even though extinct, was often placed in this division. The resemblance is more than skin deep: Psilotum (the "whisk fern") is a small

plant with prostrate rhizomes and upright stems that branch dichotomously and have an epidermis, cortex, and a simple vascular cylinder with no pith—a protostele (Fig. 2). Xylem consists of annularly or helically thickened tracheids. Psilotum is unique among living vascular plants in that it has no roots or leaves. The shoot does have occasional small projections of tissues considered to be enations. As in Rhynia, sporangia are produced at the ends of branches.

FIGURE 1: (a) Psilotum growing with mosses and ferns in a Hawaiian rainforest. (b) The growth habit of Psilotum strongly resembles that of a rhyniophyte—dichotomous axes with only a few enations. (c) Tmesipteris growing in New Zealand. (W. E. Ferguson)

FIGURE 2: (a) Vascular structure of Psilotum is simple: a protostele and phloem surrounding the xylem. Protoxylem is exarch (to the exterior of the metaxylem), which is similar to the zosterophyllophytes rather than the rhyniophytes (X 100). (b) Sporangia of Psilotum are actually considered to be three sporangia fused together; each is at the end of an extremely short branch, basically a three-branched shoot subtended by an enation (X 8).

Particularly important are the gametophytes, short, branched cylinders less than 2 mm in diameter. Their surface is covered with elongate cells, rhizoids, that act like roots and at least anchor and perhaps also absorb (Fig. 3). The gametophytes have no chlorophyll but instead are heterotrophic, forming either a symbiotic or a parasitic association with soil fungi; fungi invade most cells of the gametophyte and provide it with sugars and minerals. The only internal differentiation is that gametophytes contain a small amount of vascular tissue similar to that of sporophytes: A central mass of tracheids is surrounded by phloem and an endodermis. Psilotum is the only species whose gametophytes contain vascular tissue. A vascularized gametophyte supports the transformation theory of the origin of sporophytes. Although sporophytes and gametophytes of Psilotum are easily distinguishable from each other, they share many fundamental features.

Despite the close resemblance of Rhynia and Psilotum, fossil evidence suggests that Rhynia itself became extinct millions of years ago. No fossils of it have been found after the Middle Devonian Period, 360 million years ago. At present, most morphologists consider Psilotum and Tmesipteris to be living representatives of ancient lines, surviving remnants of the very first stages in the organization of a vascular plant, but we are not certain how closely related they are to rhyniophytes. An alternative theory postulates that Psilotum and Tmesipteris art actually highly modified relatives of ferns, and they resemble Rhynia only because of convergent evolution.

Psilotum occurs in tropical and subtropical regions. In the United States, it can be found in the Gulf Coast states from Florida to Texas as well as in Hawaii. Tmesipteris is limited to Australasia, primarily Australia and other South Pacific islands.

FIGURE 3:Gametophytes of Psilotum are small cylinders (a) that live heterotrophically, nourished by endophytic soil fungi. (b) Each gametophyte bears both antheridia and archegonia, and because the egg and zygote are retained in the archegomum, the young sporophyte initially is nourished by the parent gametophyte; during early development there is proliferation of gametophyte tissue as a protective calyptra. (c) As the sporophyte grows and enlarges, it forms an abscission zone and detaches from the gametophyte, becoming independent.