Topic: Secondary xylem (wood) in vascular plants
Evolution of wood in plants as distantly related as lignophytes, Calamites and Lepidodendron is an elegant example of convergent evolution. Plants responded in a similar way to a need for better structural support as they diversified and increased in size.
The internal transport systems of vascular plants (‘tracheophytes’) comprise specialised tissue made of elongated tubular cells, termed xylem and phloem. Xylem transports water and solutes from roots to leaves, water being essential for photosynthesis and maintaining turgor pressure. Phloem transports complex organic compounds, notably sugars, that are synthesized by photosynthesis and its subsequent reactions. As vascular plants colonised the land in the early Palaeozoic and subsequently diversified in the Devonian and Carboniferous many of them became much larger in stature and height. For plants to grow taller their transport systems had to develop improved structural support and in the case of xylem, higher conduction efficiency, so that water could rapidly reach all the way from the roots to the topmost leaves. Better support for was achieved by: (i) re-arrangement of specialised supportive tissues (e.g. parenchyma, sclereids and fibres), (ii) re-arrangement of vascular tissues into vascular bundles or cambium, and (iii) deposition of lignin as rigid secondary cell wall material in ‘metaxylem’ and ‘secondary’ xylem. Secondary xylem forms the woody tissue of shrubs and trees, and it is convergent, having evolved in the towering lycophyte Lepidodendron, in giant horse-tails (e.g. Calamites) and in the lineage containing all gymnosperms and angiosperms.
Woody lycophytes: Lepidodendron and Isoëtes
Lycophytes (plants with ‘lycophylls’ rather than true leaves) diversified during the Devonian into forms such as club mosses (Lycopodiales), Selaginellales (e.g. Selaginella), quillworts (Isoëtales) and Lepidodendrales (e.g. Lepidodendron). These plants were dominant components of tropical coal swamp ecosystems in the Carboniferous, followed by drastic reduction in diversity after more arid conditions developed in the Permian. Lepidodendron was a tree-like (‘arborescent’) tropical plant that typically reached 10-35m in height and had a straight trunk over 1m in diameter. At its crown Lepidodendron bifurcated many times, forming branches bearing clusters of small lycophylls. Lepidodendron produced a small amount of secondary xylem near the centre of its trunk, although the primary structural support for this towering plant came from an expanded bark region (composed of cortex and periderm) that encased the inner trunk tissues. Closely related to Lepidodendron we find the Isoëtales, of which the extant species Isoëtes, the quill-wort, also produces bark and some secondary xylem. Isoëtes is an aquatic or semi-aquatic freshwater plant with quill-like leaves attached to a central ‘corm’ and underground rhizome. Secondary xylem formation in Lepidodendron and Isoëtes is a feature shared from a common ancestor that appeared after divergence from lower lycophyte groups, but if we move on to consider more distant plant taxa the truly convergent nature of wood becomes clear.
Woody monilophytes: Calamites
Monilophytes are plants with true leaves (generally termed ‘euphyllophytes’) that appeared in the Devonian around 400Ma and diversified throughout the late Devonian and Carboniferous into groups that are familiar to us today, such as horsetails, seed ferns, cycads and conifers.
Horsetails (technically termed Sphenophyta) include the giant horsetail Calamites, which, like Lepidodendron, was abundant in tropical coal swamps of the Carboniferous. Calamites grew to 18m tall and had a trunk (main stem) up to 45cm in diameter, notably supported by woody secondary xylem. Horizontal whorls of branches and leaves grew at intervals (nodes) along the trunk, with an underground rhizome system at the roots. Equisetum pratense” width=”218″ height=”218″>The living horsetail Equisetum is thought to have descended from another sphenophyte genus, Equisetites, known from the Mid-Late Carboniferous (around 300Ma). Equisetum and Equisetites have a similar overall form to Calamites but are considerably smaller, herbaceous plants that lack secondary xylem altogether. This variation within the sphenophytes highlights the fact that Calamites appears to have independently acquired secondary xylem in response to selection for better support as it grew to very large proportions in its swampy environment.
A major clade of woody plants: Lignophytes
The group ‘Lignophyta’ refers to all euphyllophytes that diverged around 385Ma (Mid Devonian), namely, progymnosperms, seed ferns (pteriodosperms), cycads, conifers, Ginkgo and gnetophtytes, as well as the flowering plants (angiosperms) themselves. The name ‘lignophyte’ reflects the extensive lignin deposition in the well-developed secondary xylem or wood of all these plants. Xylem in lignophytes matures inwards from a vascular region termed the ‘bifacial cambium’, forming a wood layer of varying density depending largely on the types of xylem element present (a thin layer of secondary phloem matures outwards from this same tissue region). The innovation of secondary xylem in this lineage provided an effective mechanism to permit growth in plant size and contributed to the diversification of groups such as seed ferns, cycads, conifers, Ginkgo, gnetophytes (Ephedra, Genetum and Welwitschia) and ultimately the angiosperms.
Evolution of wood in plants as distantly related as lignophytes, Calamites and Lepidodendron is an elegant example of convergent evolution. Plants responded in a similar way to a need for better structural support while maintaining efficient vascular systems as they diversified and increased in size in their complex terrestrial environments.
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Map of Life - "Secondary xylem (wood) in vascular plants"
December 9, 2018