DESCRIPTION OF THE PHYLUM PROGYMNOSPERMOPHYTA+ (BOLD ET AL. 1987)

EUKARYA> ARCHAEPLASTIDA> VIRIDIPLANTAE> STREPTOBIONTA> EMBRYOPHYTA> TRACHEOPHYTA> PROGYMNOSPERMOPHYTA

Progymnospermophyta (pro-jim-no-sper-MA-fa-ta) is made of four Greek roots that mean before (pro -προ); naked (gymnos -γυμνός); seed (sperma -σπέρμα); and plant (phyto -φυτό). The reference is to the woody, monopodial growth habit that looks like the growth form of a conifer. The resemblance is more than superficial. The earliest seed plants likely emerged from this group.

INTRODUCTION TO THE PROGYMNOSPERMOPHYTA The progymnosperms exhibited strong monopodial (text with tooltip) Monopodial growth is characterized by extreme overtopping which produces a large central stem with smaller lateral branches. growth with the production of much lignified (text with tooltip) Lignified (adj) refers to cell walls, usually those of xylem and fiber cells, in which the cellulose wall is impregnated with lignin, a substance that serves to glue and stiffen the cellulose walls. secondary xylem (text with tooltip) Secondary vascular tissue develops from a cambium. (a.k.a. wood). From a distance these plants likely resembled spruces. Indeed, given their features shared with the conifers, the progymnosperms were the first modern trees. They flourished from the mid Devonian into the lower Mississippian when they became extinct. Aneurophyton (Figure 1) grew as a small tree with dichotomously branched (text with tooltip) Dichotomous branching is the simple pattern of branching in which each node produces two equal branches. , leaf-like branch tips. Sporangia were terminal on the ultimate branches, and the plants were homosporous (text with tooltip) Homosporous (adj) plants produce one type of spore. . Archaeopteris (Figure 2) were large trees that were major components of lowland forests from the Devonian to the lower Mississippian. These plants had simple megaphylls (text with tooltip) An megaphyll (=macrophyll) is a leaf that is derived from a lateral branch system which became webbed. Most extant vascular plants have macrophylls. on leafy branches which arose from the axils of rachial leaves on the secondary branches. Their growth habit was very conifer-like with strong monopodial growth and extensive wood. Because the plants were woody, they left extensive fossil remains where they occurred. Indeed, their fossils are quite common in Devonian strata of central Pennsylvania. The plants were heterosporous (text with tooltip) Heterosporous plants have sporangia that produce spores of different sizes: megaspores (large) and microspores (small). Megaspores produce archegoniate gametophytes, and microspores produce antheridial gametophytes. with sporangia borne on the adaxial side of the leafy branches. They seem to have been the precursors of the Pteridospermophyta, the first plants to evolve the seed habit.

SYSTEMATICS OF THE PROGYMNOSPERMOPHYTA The conifer-like vegetative morphology has long convinced the botanists (e.g. Bold et al, 1987; and Bierhorst, 1971) that the seed habit appeared in this group. Indeed, Rothwell (1999) does show Archaeopteris as the sister group to the seed plants in his analysis. Kenrick and Crane (1997b, 1996, TOL Project), Tomescu (2008), and Rothwell et al. (2009) give the orders of the progymnosperms as paraphyletic sisters to the seed plants (see Figure 3).

FIGURE 1. A reconstruction of the leafy branch of Aneurophyton. Image from http://www.historisches-centrum.de/urzeit/icons/Mdevon2_kl.jpg	FIGURE 2. A reconstruction of the habit of Archaeopteris. Note how in general form, it resembles a conifer. Image from http://biodidac.bio.uottawa.ca/thumbnails/filedet.htm?File_name=GYMN002B&File_type=GIF

FIGURE 3. Relationship between the progymnosperm orders (taxa in shaded box) and the gymnosperms. The topology is drawn from Kenrick and Crane (1997), Tomescu (2008), and Rothwell et al. 2009.

LITERATURE CITED Bierhorst, D. W. 1971. Morphology of Vascular Plants. In: N. H. Giles and J. G. Torrey. The MacMillan Biology Series. The MacMillan Co. New York. Bold, H. C., C. J. Alexopoulos, and T. Delevoryas. 1987. Morphology of Plants and Fungi. 5th Edition. HarperCollins Publishers, Inc. New York. Kenrick, P. and P. R. Crane. 1997b. The origin and early diversification of land plants: a cladistic study. Smithsonian Institute Press. Washington, D.C. Rothwell, G. W. 1999. Fossils and ferns in the resolution of land plant phylogeny. Botanical Review 65:188-218. Rothwell, G. W., W. L. Crepet, and R. A. Stockey. 2009. Is the anthophyte hypothesis alive and well? New evidence from the reproductive structures of Bennettitales. American Journal of Botany. 96(1): 296-322. Tomescu, A. M. F. 2008. Megaphylls, microphylls and the evolution of leaf development. Trends in plant science 14 (1): 5-12.

By Jack R. Holt. Last revised: 03/26/2013