THE VASCULAR CRYPTOGAMS

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INTRODUCTION TO THE VASCULAR EMBRYOPHYTES Vascular Cryptogam is an old botanical phrase, and it refers to those vascular plants that do not make seeds. Thus, cryptogam (literally hidden gametophyte) refers to the production of a separate, usually very small, archegoniate gametophyte. These are well represented in the fossil record. Kenrick and Crane (1997) report that individual spores of land plants (these could be mosses, hornworts or vascular plants) have been found in the Lower Silurian (see the Geological Time Scale), a phase in the evolution of land plants that they refer to as “Eotracheophytic”. Unequivocal evidence of vascular land plants has been found in the mid Silurian, when the first radiation of tracheophytes occurred. The early Devonian Period, however, saw an explosion in the diversity of early land plants which continued through the mid Permian, a time called the Eutracheophytic by Kenrick and Crane (1997). In fact, we know so much about the diversity and structural details of the extinct taxa that taxonomic systems of the vascular cryptogams usually include them (e.g., Bold et al. 1987). The wealth of fossil evidence from the Devonian to the present also provides the basis and support for the Telome Theory, a theory about the origins of all vascular plant organs from a simple dichotomizing axis. Pearson (1995) explored the relationships between the green plants primarily by summarizing structural and developmental evidence. His phylogenies relevant to the vascular cryptogams include a diagram of the origin and early divergence and the fern taxa. Rothwell (1999) examined 101 morphological characters of extinct and extant taxa from all major groups of vascular cryptogams. His analysis supports the phylogenetic relationships that are assumed in Pearson (1995) and Bold et al. (1987). Interpretations of relationships based on molecular evidence can be seen in a diagram from Pryer et al. (2001). Combined structure, fossil, and molecular evidence can be seen in Tudge (2000), Tomescu (2008), Rothwell et al. (2009), and the Tree of Life Project. Clearly, as indicated by Bold et al. (1987) in the case of the vascular cryptogams “Nature mocks at human categories”. With that advice, we present a taxonomic system of the vascular cryptogam phyla that roughly conforms to that of Bold et al. (1987), as modified by Kenrick and Crane (1997) and Smith et al. (2006).

FIGURE 1. MAJOR CLADES OF VASCULAR CRYPTOGAMS. This figure is adapted from Bold et al. (1987), Kenrick and Crane (1997), Pryer et al. (2001), Smith et al. (2006), Tomescu (2008), and Rothwell et al. (2009).

Major Clades of the Vascular Cryptogams. 1. Clade 1 includes all plants with axes containing vascular tissue ( xylem (text with tooltip) Xylem is vascular tissue that conducts water and functions when the cells are dead. Cell types include tracheids, xylem fibers, and vessels. and phloem (text with tooltip) Phloem is food-conducting tissue and its elements function while they are alive. Phloem cell types include sieve tubes, companion cells, and phloem fibers. ) arranged in steles. 2. The Rhyniophytes, an extinct group of plants with terminal sporangia (text with tooltip) Sporangia are spore-bearing structures. and mesarch (text with tooltip) A mesarch (adj) stele is one in which the protoxylem forms between the center and outer part of the xylem. (generally) steles that contain distinctive s-type xylem cells. 3. Plants in which overtopping (text with tooltip) Overtopping is a type of growth pattern in which there is unequal dichotomous branching. This produces a larger stem and a smaller lateral stem. occurs so that sporangia that have single lines of dehiscence, are borne laterally on an axis. The tracheids are thick and resist decay. 4. The steles are exarch (text with tooltip) An exarch (adj) stele is one that has the protoxylem on the outside of the xylem bundle. . 5. The Zosterophyllophytes, an extinct group of plants in which sporangia are associated with lateral peg-like processes on the stem axes. 6. The Lycopods are microphyllous (text with tooltip) An microphyll is a type of leaf that has a simple vascular trace going into it. The vascular trace does not produce a gap in the stele. Microphylls are characteristic of of the club mosses (Lycopodophyta). plants with adaxial (text with tooltip) This is derived from two Latin roots that mean toward (ad) the axil (axis). Adaxial is an adjective most often used to describe the leaf surface that is on top or that surface which faces the apical meristem. sporangia. 7. The Euphyllophytes are plants with endarch steles. 8. The Trimerophytes are plants with microphylls and lateral sporangia that dehisce (text with tooltip) To split open releasing spores. by longitudinal slits. 9. Plants with megaphylls. 10. The Pteridophytes (Ferns) are highly diverse plants that generally are herbaceous, rhizomatous (text with tooltip) Plants having rhizomes (horizontal stems, often underground or on the surface of the ground, bearing scale-like leaves). , megaphyllous plants with separate gametophytes. 11. The Progymnosperms and Gymnosperms plants that have strong monopodial (text with tooltip) Monopodial growth is characterized by extreme overtopping which produces a large central stem with smaller lateral branches. growth and are eustelic (text with tooltip) A eustele is a stele type characteristic of most seed-bearing plants and a few ferns and fern allies. Although other stele types can function to make wood, the eustele is the most common one. Vascular bundles characteristically are arranged in a circle around a region of pith. The cortical parenchyma is continuous with the pith through rays which separate the vascular bundles. Xylem is on the inside and phloem is on the outside of each bundle. , usually with wood.

LITERATURE CITED 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. 1997a. The origin and early evolution of plants on land. Nature. 389:33-39. Pearson, L. C. 1995. The Diversity and Evolution of Plants. CRC Press. New York. Pryer, K. M., H. Schneider, A. R. Smith, R. Cranfill, P. G. Wolf, J. S. Hunt, and S. D. Sipes. 2001a. Horsetails and ferns are a monophyletic group and the closest living relatives to seed plants. Nature. 409:618-622. 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. Smith, A. R., K. M. Pryer, E. Schuettpelz, P. Korall, H. Schneider, and P. G. Wolf. 2006. A classification for extant ferns. Taxon. 55(3): 705-731. Tomescu, A. M. F. 2008. Megaphylls, microphylls and the evolution of leaf development. Trends in plant science 14 (1): 5-12. Tudge, C. 2000. The Variety of Life, A Survey and a Celebration of all the Creatures That Have Ever Lived. Oxford University Press. New York.

By Jack R. Holt. Last revised: 04/11/2020