DESCRIPTION OF THE PHYLUM ZOSTEROPHYLLOPHYTA (BANKS 1975)
| EUKARYA> ARCHAEPLASTIDA> VIRIDIPLANTAE> STREPTOBIONTA> EMBRYOPHYTA> TRACHEOPHYTA> ZOSTEROPHYLLOPHYTA |
ZOSTEROPHYLLOPHYTA LINKS
| Zosterophyllophyta (za-ster-o-fi-LA-fa-ta) is formed from three Greek roots that mean girdle (zostra -ζώστρα); leaf (phyllo -φύλλο); plant (phyto -φυτό). It is a formalized name derived from Zosterophyllum. |
| INTRODUCTION TO THE ZOSTEROPHYLLOPHYTA These plants resembled the rhyniophytes and co-occurred with them in semi-aquatic habitats from the Silurian to middle Devonian Periods. Like the rhyniophytes, they had no roots (text with tooltip) A root is a plant axis without nodes and internodes, and it has a vascular stele that is different from that of the stem axis. or leaves (text with tooltip) A leaf is a specialized appendage that grows from a stem, and usually is the primary photosynthetic organ. Such appendages are of three types: enations, microphylls, and macrophylls (=megaphylls). , and their photosynthetic organs were their upright, photosynthetic stems (photosynthesis is inferred based on the occurrence of stomates (text with tooltip) Pores in the leaf epidermis surrounded by a pair of guard cells. The gas exchange of leaves is controlled by the dimension of the pores. . However, their steles were exarch (text with tooltip) An exarch (adj) stele is one that has the protoxylem on the outside of the xylem bundle. protosteles, and their stalked sporangia were borne singly on the sides of the axes. Some of the axes had pegs or spine-like protuberances on them, but the spines were not vascularized. In most ways the Zosterophyllophyta resemble Lycopodophyta (see Table 1), which have stalked sporangia borne on the adaxial surface of microphylls. Indeed, the microphylls of the lycopods could be considered zosterophyllophyte pegs that became vascularized. However, the the microphylls of lycopods are not just vascularized, but they also are arranged in a spiral pattern up the stem, a pattern not seen in the zosterophyllophyte enations. Furthermore, the stems of the zosterophyllophytes emerge by circinate vernation, an emergence pattern never seen in the lycopods, extant or extinct. |
 | FIGURE 1. Major Clades of the Vascular Cryptogams. P/St = polysporangiate and vascular stele O/M = overtopping and microphyll E = euphyllophyte SS = stalked sporangia |
LATERAL STALKED SPORANGIA (CLADE SS)
The appearance of lateral stalked sporangia on a stem with spines or microphylls marked the departure from the Rhyniophyta and the Trimerophytes and Euphyllophytes. The boundary between the Zosterophyllophyta and the Lycopodophyta is much less distinct. Consider Baragwanathia (Figure 4) a wetland plant from the Silurian period. It seems to straddle both the Lycopodophyta and the Zosterophyllophyta according to Table 1. Baragwanathia had spine-like leaves that may have been vascularized (making them microphylls). Similarly, the other characters are equivocal (Kenrick and Crane 1997). Now, Baragwanathia is considered to be a basal Lycopod with spine-like microphylls.
ZOSTEROPHYLLOPHYTA
The zosterophyllophytes also are defined by morphological characters in Table 1. Kenrick and Crane (1997) divide them into two major groups: those with pseudomonopodial growth and sporangia on main axes and branches (Sawdoniales) and those with simple erect axes with sporangia (Zosterophyllales). All of them seemed to have an extensive prostrate rhizome system.
ZOSTEROPHYLLALES
Wenshania (Figure 3) is a simple leafless plant from the lower Devonian of China. The interpretation of the material is difficult, but Zhu and Kenrick (1999) describe it as having flattened, reniform sporangia that emerge from a simple upright axis. The stem may be pseudomonopodial. The attachment of the sporangia to the stem is generally opposite and decussate. Zhu and Kenrick (1999) also conclude that Wenshania did not have circinate vernation. Thus, this taxon may straddle the zostero-lyco boundary as does Baragwanathia.
Zosterophyllum (Figure 4) was similar to Wenshania. They both occurred in the lower Devonian and both were simple naked axes. The attachment pattern of the sporangia also was opposite and decussate. Zosterophyllum, however, had upright axes that were unbranched. These axes that were fertile had the sporangia clustered at the tips.
SAWDONIALES
Sawdonia (Figure 5) had pseudomonopodial upright axes that were sparsely branched and covered with short spines. Most notably, they developed by circinate vernation. The sporangia on the fertile branches occurred in a spiral pattern up the stem.
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| FIGURE 2. Baragwanathia, a Silurian plant, shows a reconstruction from a fertile stem. Note the small sporangia tucked in among the spines (leaves?). Image from: http://www.ucmp.berkeley.edu/IB181/VPL/Lyco/Lyco1.html | FIGURE 3. Wenshania, from the lower Devonian of China, had reniform flattened sporangia that were decussate. The axes did not develop by circinate vernation. Image from: Zhu and Kenrick (1999) | FIGURE 4. Zosterophyllum, an early Devonian plant, has a naked axis, some sterile and some fertile (A). A detail of the stem (B) shows that it had a protostele. Image from: http://www.ucmp.berkeley.edu/IB181/VPL/Lyco/Lyco1.html | FIGURE 5. Sawdonia, also from the early Devonian, has an axis with stout spines and some fertile branches. Note that some overtopping occurs in this plant (A). The detail of the stem shows the protostele,
eusporangia
(text with tooltip)
A eusporangium is the most common spore-bearing structure in plants. Eusporangia develop from more than one cell and usually have a wall of several cell layers. Contrast a eusporangium with a leptosporangium.
with lateral
dehiscence
(text with tooltip)
Dehiscence refers to way in which a sporangium (or other fruiting structure) splits to release its contents.
, and stout spines (B). Image from: http://www.ucmp.berkeley.edu/IB181/VPL/Lyco/Lyco1.html |
| TABLE 1. Comparisons between the Zosterophyllophyta and the Lycopodophyta (Kenrick and Crane 1997). | ||
| CHARACTER | ZOSTEROPHYLLOPHYTA | LYCOPODOPHYTA |
| Leaf | no vascularized microphylls; enations or pegs are arranged with no pattern on the stem | microphylls present and arranged in a helical pattern |
| Stele | stele is a circular or oval protostele (as seen in cross-section) | stele usually actinostelic |
| Stem Emergence | circinate vernation | no circinate vernation |
| Sporangium arrangement | some are arranged oppositely on the stem | most show helical arrangement |
| SYSTEMATICS OF THE ZOSTEROPHYLLOPHYTA Banks (1968) defined the Zosterophyllophyta as a group distinct from the Rhyniophyta. Bierhorst (1971) and to a certain extent Bold et al. (1987) suggested that the zosterophyllophytes represented early forms of the lycopods. Although Kenrick and Crane (1997) support the relationship between the groups and consider them to occupy a clade called Lycophytina, they do see the Zosterophyllophyta and the Lycopodophyta as sister groups. |
| LITERATURE CITED Banks, H. P. 1975. Reclassification of Psilophyta. Taxon. 24: 401-413. 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. Doyle, J. A. 1998b. Phylogeny of vascular plants. Annual Review of Ecology and Systematics. 29:567-599. Kenrick, P. and P. R. Crane. 1997b. The Origin and Early Diversification of Land Plants: A Cladistic Study. Smithsonian Institute Press. Washington, DC. Northington, D. K. and J. R. Goodin. 1984. The Botanical World. Times Mirror/Mosby College Publishing, St. Louis. Zhu, W.-Q. and P. Kenrick. 1999. A Zosterophyllum-like plant from the lower Devonian of Yunnan Province, China. Review of Palaeobotany and {alynology. 105: 111-118. |
| By Jack R. Holt. Last revised: 03/25/2013 |
