Progymnosperms : a brief illustration

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Progymnosperms : a brief illustration 

What Progymnosperms Mean:

It is currently believed that some fossils from the Devonian and Carboniferous eras of the Paleozoic era, which were formerly believed to belong to pteridophytes (Primofilices), are more closely related to gymnosperms even though they are not fully gymnospermous.

Following the discovery of similarities between the fern-like fronds of Archaeopteris and the gymnosperm-like trunks of Callixylon, these fossils were categorized by Beck (1960) as belonging to the Progymnospermopsida class of gymnosperms. Progymnosperms are defined as "plants exhibiting the features of gymnospermic anatomy and ptendophytic reproduction," according to Bonamo (1975).




 From the Middle Devonian through the Lower Mississippian, prosims were an essential part of the vegetation. They were somewhat similar to gymnosperms, as their name suggests. Similar to true gymnosperms, progymnosperms also frequently developed wood by secondary growth of their vascular tissues, and some of them even reached the height of tall trees. In contrast to gymnosperms, they dispersed their spores like ferns instead of producing seeds. While some progymnosperms produced a large number of identical spores, others were heterosporous, meaning they produced two distinct types of spores. It is believed that the seed plants' progenitors, or at least near cousins, are this latter group.

There are two known major groups of progymnosperms: the Aneurophytales and the Archaeopteridales. The Protopityales, a third group, is occasionally identified but is not well understood, making it difficult to determine how it relates to other plant species.

Upper Devonian to Mississippian Archaeopteridales
Charles B. Beck's work from the 1960s is arguably one of the most notable paleobotanical findings. He was able to prove, by laboriously thorough investigation, that the leaves, called Archaeopteris, on the left, and the fossil wood, called Callixylon, were genuinely from the same plant. This might not seem like much until you learn that whole organisms are rarely preserved in the fossil record.


It was discovered that the plant now known as Archaeopteris was truly an extinct group of plants unlike any living plants today—it had both wood and fern-like reproduction—when Dr. Beck showed how fossils of Archaeopteris were physically bonded to Callixylon wood.


Progymnosperm xylem, on the other hand, has circular-bordered pits that resemble conifer xylem but are not quite the same. As a result, the suggested tight link between conifers and progymnosperms has been reexamined by several paleobotanists.

In the northern hemisphere, Devonian strata are home to Archaeopteris, which has been documented from Australia. Recreated as a massive coniferous tree akin to a redwood or arborvitae, it is thought to have grown to a maximum height of 20 meters. This group's other genera are less.


(Middle to Upper Devonian) Aneurophytales
As the most basic progymnosperms, aneurophytes seem to have evolved in an evolutionary transitional between plants belonging to the "trimerophyte" grade and heterosporous progymnosperms such as Archaeopteris.
Middle and Upper Devonian rocks include Aneurophyton, the most prevalent member of the Aneurophytales. This plant exhibits three-dimensional branching with branches arranged in a spiral or decussate pattern, similar to some trimerophytes as Pertica. Aneurophyton possessed a protostele, which was typically lobed, just like the trimerophytes. However, the aneurophytes added wood to their stem and may have grown into bush-sized plants or extensive vines. They function as a bridge between the bigger, tree-sized progymnosperms like Archaeopteris and the smaller, mostly herbaceous trimerophytes.

Aneurophytes are distributed by spores, just like other progymnosperms. Away from the leaves, in the fertile areas of the branches, were clusters of sporangia. While the majority of aneurophytes were homosporous, Tetraxylopteris was one genus that produced spores with an exceptionally wide variety of sizes. It's possible that this plant was headed toward heterospory.

Progymnosperms have long been thought to have played a significant role in the genesis of seeds. Paleobotanists who study heterosporous Middle Devonian plants, such as progymnosperms, are of the opinion that the earliest seed plants originated from them. Heterospory appears to be a reasonable transitional stage since the "seed habit" starts with each megasporangium being reduced to a single functioning megaspore. This would suggest that the ancestors of seed plants are Archaeopteris and its relatives. 

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