A Simple Type of Wood in Two Early Devonian Plants

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Science  12 Aug 2011:
Vol. 333, Issue 6044, pp. 837
DOI: 10.1126/science.1208882


The advent of wood (secondary xylem) is a major event of the Paleozoic Era, facilitating the evolution of large perennial plants. The first steps of wood evolution are unknown. We describe two small Early Devonian (407 to 397 million years ago) plants with secondary xylem including simple rays. Their wood currently represents the earliest evidence of secondary growth in plants. The small size of the plants and the presence of thick-walled cortical cells confirm that wood early evolution was driven by hydraulic constraints rather than by the necessity of mechanical support for increasing height. The plants described here are most probably precursors of lignophytes.

Wood (secondary xylem) derives from a lateral meristem called vascular cambium. It is considered essential for production and maintenance of trees but is equally important in hydraulic functioning of smaller growth forms. Wood includes longitudinally elongated cells (water-conducting elements and fibers) and transversally oriented cells (rays); it is characterized by radial rows of cells resulting from the periclinal divisions of cambium initials. A vascular cambium evolved independently in several Devonian and Carboniferous plant lineages [supporting online material (SOM) text]. Unlike the cambium initials of all other lineages, the cambium initials of the lignophytes (progymnosperms and seed plants) frequently divide anticlinally (1), which results in circumferential growth. The Middle Devonian (397 to 385 million years ago) aneurophytalean progymnosperms are currently considered the earliest plant lineage with wood (2). An earlier origin of wood is generally considered unlikely because Early Devonian plants were all herbaceous (SOM text).

We report here two taxa of plants from two Early Devonian localities: a late Pragian/earliest Emsian (~407 million years ago) locality from France and a latest Emsian (~397 million years ago) locality from Canada (SOM text). Specimens are pyritized stem portions up to 5 mm wide and 12 cm long. Their anatomy is consistent with basal euphyllophyte affinities; both plants are close to the genus Psilophyton (SOM text). The plants exhibit a circular or elongate primary xylem strand (Fig. 1, A and B). In some axes, thick-walled cells comprise an outer cortical region (Fig. 1B). In many specimens, the primary xylem is surrounded by wood. The tracheids are placed in radial rows (Fig. 1, A to D, and fig. S1); they are generally rectangular in transverse section. At several places in all specimens (Fig. 1, C and D, and fig. S1), two xylem cell rows emanate from a single row. Tracheids with smaller radial diameter are common at the periphery (fig. S1). In some sections, possible remains of the vascular cambium or of its developing derivatives are observed (Fig. 1E).

Fig. 1

(A) Transverse section of Châteaupanne plant (CP), sample CP9-1-2-B2. (B) Transverse section. Thick-walled cortical cells at arrows. New Brunswick plant (NBP), NBP-NCUPCS54. (C) Enlargement of (A). Rays are at black arrows; doubling of xylem cell rows at white arrow. CP9-1-2-B2. (D) Detail of wood; transverse section. Ray at black arrow; doubling of xylem cell rows at white arrow. NBP-NCUPCS54. (E) Possible remains of vascular cambium at arrow; tangential section. CP31-1-CL18. (F) Ray; tangential section. CP16-1-CL5. (G) Ray; radial section. CP31-1-CL25. Scale bars, 100 μm, except for (F), 10 μm.

In all sections, spaces surrounded by only one cellular wall are found dispersed among the radially aligned tracheids (Fig. 1). The single cell wall surrounding those spaces belongs to adjacent tracheids (fig. S2). The single-walled spaces are interpreted as rays in which the individual cells were not preserved. Their variable shape, size, and extension (fig. S3) compare well to patterns of ontogeny exhibited by early differentiated ray cells in extant gymnosperms (3).

The identification of a derived anatomical feature such as wood in two Early Devonian plants was unexpected. The secondary xylem of the plant from France predates, by at least 10 million years, other early occurrences of wood. The small size of both plants and the presence of thick-walled cells in their cortex support the earlier suggestion (4) that the evolution of wood was initially driven by hydraulic constraints rather than by the necessity of mechanical support for increasing height. This is consistent with the large diameter of the wood tracheids, which improves conducting capacities, and with the need for increased conductance, resulting from the decrease of atmospheric carbon dioxide concentrations during Early Devonian times (5).

The doubling of several secondary xylem cell rows indicates that radial longitudinal (anticlinal) divisions of cambial initials were frequent. This is typical of lignophytes. Even though the character might be homoplasious, its presence in the two studied plants suggests that the latter are lignophyte precursors or early representatives of that lineage.

Supporting Online Material

SOM Text

Figs. S1 to S3

References (618)

References and Notes

  1. Acknowledgments: We thank B. Meyer-Berthaud, M. Streel, and D.-G. Strullu for constructive comments. Material is housed in the paleobotanical collections of ULg and UNC. P.G. is a Fonds de la Recherche Scientifique –Fonds National de la Recherche Scientifique (F.R.S.-FNRS) Research Associate; P.S. is a F.R.S.-FNRS Senior Research Associate.
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