Cycads and Gnetophytes

Abstract

The cycads and gnetophytes, which make up the subdivision Cycadophytina, represent relicts (“living fossils”) of gymnospermous groups which had their main expansion in Jurassic and Cretaceous times. Consequently, their extant species (some 100 in the cycads and about 70 in the gnetophytes) are regarded as the specialized remnant of a once very successful line which in the Devonian has the same origin as the Coniferophytina, i. e., in the so-called Progymnospermae. Since this connection is mediated via the seed ferns in upper Carbonian/lower Permian times, a group to which the Angiospermae are also linked, the Cycadophytina have often been regarded as connecting links between gymnosperms and angiosperms. Gnetum, in particular, seems to be well suited for this role since, unlike all other gymnosperms, this genus has been shown to include vessels in its xylem and at some time was also thought to contain companion cells in its phloem. The few ultrastructural investigations of the phloem in this group available, however, corroborate the view presently held by taxonomists that the gnetophytes represent a highly specialized but distinct gymnospermous taxon.