Rather than plants, it was
fungi, in particular
nematophytes such as
Prototaxites, that dominated the early stages of this terrestrial biodiversification event. Nematophytes towered over even the largest land plants during the Silurian and Early Devonian, only being truly surpassed in size in the Early Carboniferous. The nutrient-distributing glomeromycotan mycorrhizal networks of nematophytes were very likely to have acted as facilitators for the expansion of plants into terrestrial environments, which followed the colonising fungi. However, recent research suggests that
Prototaxites may not have been a fungus after all but part of a completely different clade of eukaryotes. The first
fossils of
arbuscular mycorrhizae, a type of symbiosis between fungi and vascular plants, are known from the Early Devonian.
Lichens such as
Spongiophyton were widespread and had outsized ecological prominence during the Early Devonian, just before the development of forests. Land plants probably evolved in the Ordovician. The earliest
radiations of the first land plants, also known as
embryophytes, were
bryophytes, which began to transform terrestrial environments and the global climate in the Ordovician.
Baltica was a particularly important cradle for early land plant evolution, with it having a diverse flora by the
Darriwilian. ∆199Hg and ∆200Hg excursions reveal that land plants had already spread across much of the Earth's land surface by the
Early Silurian. The end of the
Homerian glaciation, a glacial phase of the
Early Palaeozoic Ice Age, and the corresponding period of global warming marked the first major diversification of plants that produced trilete spores. The later glaciation during the middle
Ludfordian, corresponding to the
Lau event, led to a major
marine regression, creating significant areas of new dry land habitat that were colonised by plants, along with cyanobacterial mats. These newly created terrestrial habitats helped facilitate the global expansion and
evolutionary radiation of
polysporangiophytes. A warming climate during the subsequent
Pridoli epoch lent itself to further floral diversification. During the
Wenlock epoch of the Silurian, the first
fossils of
vascular plants appear in the fossil record in the form of
sporophytes of
polysporangiophytes. Lycophytes first appeared during the later
Ludlow epoch in the form of
Baragwanathia, which was an aquatic predecessor of fully terrestrialised lycophytes.
Palynological evidence points to Silurian terrestrial floras exhibiting little provincialism relative to present day floras that vary significantly by region, instead being broadly similar across the globe. Plant diversification in the Silurian was aided by the presence of numerous small, rapidly changing
volcanic islands in the
Rheic Ocean that acted as natural laboratories accelerating evolutionary changes and enabling distinct, endemic floral lineages to arise. Silurian plants rarely reached large sizes, with heights of 13 cm, achieved by
Tichavekia grandis, being exceptionally large for the time. The Devonian witnessed the widespread greening of the Earth's surface, with many modern vascular plant clades originating during this period.
Basal members of
Euphyllophytina, the
clade that includes
trimerophytes,
ferns, progymnosperms, and
seed plants, are known from
Early Devonian fossils.
Lycopsids experienced their first evolutionary radiation during the Devonian period. although zosterophyllopsids displayed high levels of endemism. In the Middle Devonian, euphyllophytes continued to increase in diversity. The first true forest environments featuring trees exceeding eight metres in height emerged by the Middle Devonian, with the earliest known fossil forest dating to the
Eifelian. The oldest known trees were members of the clade
Cladoxylopsida. Devonian swamp forests were dominated by giant horsetails (
Equisetales), clubmosses, ancestral ferns (
pteridophytes), and large lycophyte vascular plants such as
Lepidodendrales, referred to as scale trees for the appearance of scales on their
photosynthetic trunks. These lycophytes, which could grow up to 40 metres high, grew in great numbers around swamps along with tracheophytes.
Pseudosporochnaleans (morphologically similar to palms and tree ferns) likewise experienced a similar rise to dominance. Archeopteridaleans had likely developed extensive root systems, making them resistant to drought, and meaning they had a more significant impact on Devonian soil environments than pseudosporochnaleans. The Late Devonian saw the most rapid land plant diversification of the Devonian, largely owing to the rapid radiation of pteridophytes and progymnosperms. Cladoxylopsids continued to dominate forest ecosystems during the early Late Devonian. During the latest Devonian, the first true spermatophytes appeared, evolving as a sister group to archaeopteridaleans or to progymnosperms as a whole. This epoch was characterised by a trend of floral homogenisation, with exchanges between regions such as South China and Gondwana contributing to a high degree of uniformity of plant communities. Most flora in Devonian coal swamps would have seemed alien in appearance when compared with modern flora, such as giant horsetails which could grow up to 30 m in height. Devonian ancestral plants of modern plants that may have been very similar in appearance are ferns (
Polypodiopsida), although many of them are thought to have been
epiphytes rather than grounded plants. True gymnosperms like ginkgos (
Ginkgophyta) and cycads (
Cycadophyta) would appear slightly after the Devonian in the
Carboniferous. Vascular plant lineages of sphenoids, fern,
progymnosperms, and seed plants evolved laminated leaves during the Devonian. Plants that possessed true leaves appeared during the Devonian, though they may have many independent origins with parallel trajectories of leaf morphologies. Morphological evidence to support this diversification theory appears in the
Late Devonian or
Early Carboniferous when compared with modern leaf morphologies. The
marginal meristem also evolved in a parallel fashion through a similar process of modified structures around this time period. In a 1994 study by
Richard M Bateman and William A. Dimechele of the evolutionary history of
heterospory in the plant kingdom, researchers found evidence of 11 origins of
heterospory events that had occurred independently in the Devonian within
Zosterophyllopsida,
Sphenopsida,
Progymnospermopsida. The effect of this heterospory was that it presented a primary evolutionary advantage for these plants in colonising land. Early land plants had conducting systems consisting of a single type of cell able to transport solutes and water alike. The simultaneous colonisation of dry land and increase in plant body size that many lineages underwent during this time was likely facilitated by another parallel development: the replacement of the ancestral central cylinder of
xylem with more elongate, complex xylem strand shapes that would have made the plant body more resistant to the spread of drought-induced
embolism. Early vascular thickenings, based on analysis of the Rhynie Chert flora, appear to have been unlignified.
Tracheids, tapered cells that make up the xylem of vascular plants, first appear in the fossil record during the Early Devonian. Evidence of root structures appears for the first time during the Late Silurian. Further appearances of roots in the fossil record are found in Early Devonian lycophytes, and it has been suggested that the development of roots was an adaptation for maximising water acquisition in response to the increase in aridity over the course of the Silurian and Devonian. The Early Devonian also saw the appearance of complex subterranean
rhizome networks. == Effect on atmosphere, soil, and climate ==