Aglaophyton is a genus of early plants from the Pragian stage of the Lower Devonian (~410 mya). It was a small, diplohaplontic species of terrestrial plant, meaning that it had a life cycle that alternated between sporophyte and gametophyte stages (this is the method by which land plants undergo sexual reproduction). Aglaophyton had morphological features that labeled it as an intermediate between the bryophytes (non-vascular plants) and tracheophytes (vascular plants). Non-vascular plants do not use a xylem and phloem for the transport of water whereas vascular plants do. Examples of extant non-vascular plants include liverworts. Aglaophyton were free-sporing, meaning it had no pollen or seeds and the spores are the way the plant disperses. It grew in a mat of horizontal vines that had terminal sporangia (the spore forming organs) that turned upwards into the air (Remy & Remy, 1980; Edwards, 1986; Remy & Hass, 1996; Taylor et al. 2008).
The fossil remains of this species are restricted to the Rhynie chert in Aberdeenshire, Scotland. During the Lower Devonian, this area’s environment was dominated by hot springs and silica rich substrates somewhat like parts of Yellowstone National Park. This location was a hot bed of early plant evolution, as Aglaophyton was found here with a number of early vascular plants and Asteroxian mackei, the precursor to modern Lycopsida, or clubmosses (Edwards, 1986; Powell et al., 2000a; Powell et al., 2000b).
The plant, Aglaophyton, attained a height of around 18 cm and the vine-like axes (much like a plant stem, but can be situated in both vertical or horizontal positions) of Aglaophyton possessed a maximum diameter of 6 mm. Axes is a term used to refer to branching parts of early plants that are not considered true stems. This is, in part, because true stems rise vertically into the air. Aglaophyton grows in a patchy carpet of axes that grow in a creeping pattern across the ground and occasionally turn upwards. The individual axes are bifurcating, meaning they divide into two branches (the angles between the two branches range between 60 and 90 degrees). As with all plants, morphology is not strict and variation is the rule not the exception and, therefore, occasional adventitious branching does occur within the genus (Edwards, 1986).
The vine-like axes that run along the ground and function like rhizomes. In modern plants, rhizomes are modified underground stems that send out roots and shoots from its nodes. Ginger root is an example of a rhizome. The ground-laying rhizomes that comprise Aglaophyton are cylindrical, nude, and generally exhibit a similar outer-morphology and internal anatomical arrangement to the upright aerial axes. Both portions of the plant bear stomata (a pore that enables gas exchange in plants). As mentioned, Aglaophyton are comprised of creeping rhizomes that lay directly upon the ground and regularly turn upwards and into the air (Edwards, 1986, Taylor et al. 2008).
The sporangia (the reproductive organ that produces spore) of Aglaophyton are large (maximum size of 12 mm by 4mm), elongated and fusiform (tapering at both ends) in shape. The sporangia are located in pairs on terminal positions upon the aerial branches (Edwards, 1986; Remy & Hass, 1991).
The free-living gametophyte phase of Aglaophyton consists of an aerially positioned branch that widens and ends in a relatively large, cup-like structure that bears the antheridia (organ that produces and stores male gametes, or sperm). The gametophyte is smaller in size than the sporophyte condition of this plant, yet very similar in their anatomy (Remy & Remy, 1980; Edwards, 1986).
Aglaophyton was a common plant in the Rhynie Chert ecosystem during the Lower Devonian (Pragian stage, ~410 mya). It grew on detritus (decomposing organic matter) covered surfaces and was therefore never a pioneer species of unexploited habitat (Edwards, 1986).
Aglaophyton most likely grew in large surface patches, as indicated by its creeping rhizomal axes. Forming a loose blanket of vegetation, it occasionally grew in isolated groupings, but more is more often found in association with other early plants (such as Nothia, Asteroxylon, Horneophyton and Rhynia) (Powell et al., 2000a).
Aglaophyton possesses stomata (an anatomical feature that allows oxygen exchange and the process of transpiration in plants) on the rhizomal axes (ground-laying axes) as well as the vertically oriented aerial axes, indicating that the plant inhabited dry substrates. There is also evidence found within the cuticle and stomata that suggest that Aglaophyton could tolerate periods of environmental desiccation, or drought (Powell et al., 2000b).
Despite being adapted to a generally dry environment, there is some indication that occasional flooding occurred within the Rhynie Chert beds and may have been necessary for the germination of Aglaophyton spores (Remy & Hass, 1996).
Life History and Behavior
Aglaophyton possesses a diplohaplontic life cycle (an alternation of phases or generations). This is where a haploid (n chromosomes, unpaired chromosomes) multicellular gametophyte alternates with a diploid (2n chromosomes, or paired chromosomes), multicellular sporophyte. Once mature, a sporophyte produces spores through the process of meiosis, where the number of chromosomes is cut in half, from 2n to n. Meiosis is a critical process in the alternation of generations and it is thought to be a key adaptive function. There are two main ideas on the true function of meiosis; it is thought to facilitate the repair of damaged DNA and/or create genetic variation (Remy & Remy, 1980; Remy & Hass, 1991; Williams, 2009). This alternation of generations is the method by which land plants undergo sexual reproduction.
Names and Taxonomy
Kidston and Lang first described Aglaophyton major as Rhynia major in 1920 (Kidston & Lang, 1920). It wasn't until 1986 that a researcher did a follow up examination of the fossil specimens and discovered that they lacked true vascular tissue, but had conducting tissue that was similar to that of bryophytes (Edwards, 1986, Taylor et al. 2008). The lack of vascularization led to the creation of the genus Aglaophyton, to contain the species originally described as Rhynia major (Edwards, 1986).
Later, researchers published a cladogram for the polysporangiophytes that places Aglaophyton as a sister to the tracheophytes, or vascular plants, with the Horneophytopsida being sister to both. The cladogram is structured this way because the conducting tissue in Aglaophyton is more developed than Horneophytopsida, but still lacks true vascular tissue (Crane et al., 2004).
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