5708 - Plants in extreme detail

To support science I belong to the AAAS (American Association for the Advancement of Science). With that membership comes a magazine called Science. It comes every week. It shows a price of $15 for each magazine. The AAAS membership for a year is $55. 52 X $15 = $780 worth of magazines for $55. 

There has to be a catch, right? Well, there is. Unless you have a post-doctorate in what a particular article is about good luck understanding it.

Here is part of an article about plants titled Hydraulic failure as a primary driver of xylem network evolution in early vascular plants. It's behind a paywall. This is part of the article plus all the pictures (I like pictures.). If you want to see the whole thing, email me.

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Drought shapes plant architecture

Since plants colonized land, they have developed increasingly complex vessel architectures to carry water from their roots to their highest leaves. Vascular plants now display a diversity of xylem strand shapes in cross section, from elliptical to linear to many lobed. Bouda et al. investigated whether selection from drought, which causes vessel cavitation and embolism, drove the complexity of xylem strand shape as plants inhabited drier climates. By simulating embolism spread between vessels across varying shape and complexity, including those seen in extant lycophytes and ferns and extinct plant fossils, the authors found that evolutionary changes in xylem strand shape have reduced embolism spread and made plants less vulnerable to drought. —BEL

Abstract

The earliest vascular plants had stems with a central cylindrical strand of water-conducting xylem, which rapidly diversified into more complex shapes. This diversification is understood to coincide with increases in plant body size and branching; however, no selection pressure favoring xylem strand-shape complexity is known. We show that incremental changes in xylem network organization that diverge from the cylindrical ancestral form lead to progressively greater drought resistance by reducing the risk of hydraulic failure. As xylem strand complexity increases, independent pathways for embolism spread become fewer and increasingly concentrated in more centrally located conduits, thus limiting the systemic spread of embolism during drought. Selection by drought may thus explain observed trajectories of xylem strand evolution in the fossil record and the diversity of extant forms.

Water availability is a critical limiting factor for land plants (1), whose macroevolution is marked by a series of hydraulic milestones that mitigated water loss, provided control over transpiration, and increased water transport efficiency (2, 3). These adaptations mark distinctions between major land plant lineages (2), each releasing plants from hydraulic constraints and thereby enabling them to expand their niche space into drier environments (4). The earliest tracheophytes had a simple cylindrical (terete), centrally located vascular strand [stele (5)] containing xylem with tracheids as the water conducting cells (6). This terete haplostele shape occurs repeatedly early in the fossil record, but steles soon diversified toward more radially elongated shapes or larger, more elaborate forms (Fig. 1A) (3, 5, 6). Despite a century-long debate over the evolutionary drivers of stele complexity from the Devonian and Carboniferous periods onward (3, 7–10), no underlying selective pressure has been found to account for the observed patterns. Presently, the dominant view is that changes to vascular architecture were a developmental artifact of increasingly branched or complex plant bodies (3, 11, 12).