Evolution of H2O Conservation Methods in "Equisetum"
From as far back as the Devonian, fossil predecessors of Equisetum have shown progressive methods of water conservation. Presumed to have originated during a fairly wet period of Earth's history, the repetitive cycling of moist and dry conditions in the areas where Equisetum ancestors flourished enabled the evolution of several family-specific characteristics. These include thickened cells constituting the outer walls, the fusion of leaves into a sheath surrounding the main stalk of the plant, and the incorporation of silica into the stems.
Phylogenetic research into the origins of Equisetum has lead to a probable first ancestor. It is now hypothesized that the Trimerophytophyta were the family from which all sphenophytes are derived. Arising in the Devonian era, trimerophytes consisted mostly of wholly photosynthetic branching stems, lacking all root and leaf structures. Their branching was unequal, leading to the appearance of a main axis with several smaller lateral offshoots. Vascular tissue had evolved by now, forming a protostele within the trimerophytes (Stewart, 210). All of the trimerophyte vascular systems are composed of a three-ribbed protostelic system throughout with the exception of the extreme appendages. The vascular tissues begin in the center of the stem and grow outward. When one of the strands crosses an area where a lateral branch occurs, it begins to enlarge in the direction of the branch, resulting in the ribbing of the stem. (Wright, 209).
The arrangement of the branches in a trimerophyte is unique. In the plant families ancestral to it, all extensions arise as a dichotomously branching. The trimerophytes brought about a new method. Lateral branches arise in a fashion similar to the whorled leaves of the modern horsetail with a notable exception. The branches stem out from the main axis in a "staircase" arrangement, the lateral extensions coming off of the stem at one point with the next one growing a certain distance around the stem and slightly higher up (Stein, 108).
In the middle Devonian, Trimerophytophyta gave rise to Ibyka, a cryptic genus found in the eastern parts of the United States. Morphologically, Ibyka is similar in appearance to the trimerophytes. The plant is composed of the same sort of branching system with the lateral branches arising from the main stem in a spiral form. It's been determined that the width of the spiral arrangement was a function of the branching within the xylem (Skog, 368).
The xylem branching in Ibyka is seen as the next evolutionary step in the history of Equisetum. Here, the stele no longer exists as a single strand or column of vascular tissue. Instead, it begins forming branches, usually six in number. It's been thought that the six branches are actually only three appendages that have undergone dichotomous branching. Sclereids first appeared, the thickened cell walls seemingly in association with the ridges of the plant (Skog, 370).
Sometime in the Devonian, the exact time is unsure, there came Cladoxylopsida, a fern-like plant. Most were less than 30 cm in height with a diameter of 3 cm. Morphologically, they resembled many modern woody shrubs, consisting of a central trunk with several branching members. It was in this family that the first stages of a complex vascular system are visible, as a series of scattered xylem vessels. (Berry, 350.) Most importantly, it is the development of the mestarch protoxylem that links Cladoxylopsida with the horsetails (Taylor, 93). Unlike most xylem fibers, the mestarch protoxylem develops first in the interior of a plant and grows outward.
A problem arises that the Cladoxylopsida did not seem to produce leaves of any sort, though it is difficult to be certain from the fossil impressions and casts that have been retrieved to date. That would mean that the descendants of Cladoxylopsida, both the ferns and the horsetails, may have evolved leaves independently of each other (Taylor, 15).
Plant Genus Adaptation
Trimerophytes Spiral branching pattern
Cladoxylopsida Mestarch protoxylem
Ibyka Branched stele, sclereids
Pseudosporochnus Special sporangia-bearing branches
Calmephyton/Hyenia Whorled branches
Table 1: Evolutionary adaptations contributing to Equisetum
Pseudosporochnus came about shortly after the Ibyka in the Devonian and has been the subject of much scrutiny in the reconstruction of the phylogeny of the horsetails. The superficial morphology seems reminiscent of the fern fronds. Characteristically, Pseudosporochnus is defined by a thickened central axis covered by depressions (Berry, 350), probably giving it a scaly bark appearance with its three meter height. The spirally arranged lateral appendages spread out in a main finger-like branching pattern, the extremities dividing even further with the sporangia clustered on the tips (Berry, 353). Internally, the primary xylem again shows the dissection found previously in Ibyka with some advancement on the separation of branches towards the ribs (Stein, 107).
The change in the sporangia accounts for the next link in the phylogeny. Pseudosporochnus marks the starting point for sporangia to reside on branches seemingly specialized precisely for that task (Stein, 107).
Calmaphyton and Hyenia are usually spoken of in the phrase due to their overwhelming similarities. Both genuses flourished in the Carboniferous in the areas around the tropics. In appearances, they looked a great deal like modern Equisetum. Growing in height up to 10 meters, the main stem arose from a rhizomous roots system with the typical nodes dispersed along its length (Cleal, 45). Along with its ancestors, the interior morphology contained a branching, dissected vascular system that possessed ribbing leading towards each branching offshoot of the main axis. However, these plants were also able to produce a secondary wood in their structures, making them very treelike (Taylor, 51).
What has lead researchers to classify Calmaphyton and Hyenia as the most recent ancestors of the modern horsetail, including the main morphological characters mentioned for the previous groups, is the external branch arrangement. Whorls of leaves grew on the branches off the main trunk (Stein, 110 ), leading to the modern horsetail as we know it.
Scientists know that much of the anatomy of the modern horsetails has been derived from a need to stave off water loss. The usual environment of these plants is in damp, nutrient deficient soils such as lakeshores and swamps. However, during their evolutionary history, there would have ancestrally been many times when moisture would be difficult to find and the ability to store it been tantamount.
Global paleoclimatology plays a key figure in understanding why these groups evolved as they did. Analysis into the preserved tree ring and the thickness of coal deposits from the Carboniferous Period, the time when several of Equisteum's ancestors were proliferous, has yielded a good model of the climate. Over the 74 million year expanse of time, the Earth went from one extreme to another.
The early part of the Carboniferous appears to have been very similar to the mental image people have of the ancient world. Giant tree-like ferns growing in a year-round moist, sweltering environment along the lines of the rain forests. Climate data has shown that average Earth temperatures were much higher than the present, ranging around 22 C (72 F). With these temperatures was a steadily rising sea level worldwide as the water levels rebounded from the previous glacial period. This was the time when Pseudosporochnus and some species of Calamophyton were in abundance.
As time progressed towards the middle of the Carboniferous, worldwide temperatures began to drop at the onset of another glacial advance. Former intercontinental seas, the most likely environment for ancestral horsetail, began to dry up or freeze over, depending on their location. Toward the very end of the Carboniferous, the majority of the world experienced desert conditions in areas where it wasn't covered by a mile of ice. Evolution towards a more water conservative system became a necessity.
The biggest factor in determining the climatological push for methods of water conservation comes from the study of the Carboniferous coal beds. Studies done in India have shown that many of the climate zones switched towards a more mid-latitude temperate one during the glaciation. These "temperate" climates would have existed in the poles due to the geography of the continental plates residing on the equator. All of the areas immediately lying along the equatorial line were bleak arid environments, so shown by the vast evaporite deposits (Blackett, 19).
As the period drew to a close, the fluctuations between wet and dry conditions increased. An area that had been a swamp could easily become a Death Valley-like expanse within just a few years. The reverse is also true as flooding from brief melt periods in the glaciers impacted the entire globe (Blackett, 23).
Though we know that Equisetum evolved to incorporate silica into its framework in order to deter water loss, scientists have yet to find the evolutionary link showing where that adaptation came to be. The main reason for that comes from the issue of fossilized horsetail ancestors undergoing quartz replacement in their stems. Once this occurs, there is no way of knowing what silica became emplaced after fossilization and what came from the plant itself.
In order to resolve this situation, finding a series of fossils that haven't undergone this replacement would be necessary. The best chance of finding these would most likely be in a deep coal seam, one that hasn't been exposed to water in recent geologic history and has been spared from mining to date. Should one of these be found, a simple microscope analysis should be sufficient to determine the structure of the supports in the plant stem. Silica has a very determinate birefringence under cross-polarized light and is easy to recognize. Given the geologic conditions that helped to spur the horsetail evolution, its quite possible that silica incorporation came about in the latter Carboniferous when water was at a premium and those that survived knew how to keep it.
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