HOW WOOD EVOLVES
If the title of this 2012 seems to imply that I am solving all problems in wood evolution---well, I'm not, of course. But I'm asking some questions where the answers are less clear. We do know a great deal, and an Editor of Botany (formerly Canadian Journal of Botany) invited a general paper on this subject, and somehow I was ready to write one. I did a lot of growing in the process—much of the literature cited is in plant physiology, and I feel that structure and function should be intercontinuous in the way we approach them. So the paper was a great learning opportunity for me. Often, today, scientists have such time constraints that they can't sit down and learn the basic literature in a field. And admittedly, there is more and more basic literature to learn in any field of science. Unfortunately, scientists are not rewarded for doing interdisciplinary work—if anything, going interdisciplinary creates doubt that one is competent in any given area. Renaissance men might have been valued in the Renaissance, but Specialists are valued now. And yet, advances in science come from interdisciplinary work, as Darwin proved in "On the Origin of Species...", much more than from intensive attention to particular phenomena.
When one reconsiders patterns of wood evolution, as I did in the 2102 paper, "How Wood Evolves...," how does one do it? Some will view the 2012 paper as a review. However, I think of it more as an attempt to construct a new theoretics of wood evolution, together with supporting data (the clade Campanulidae is used extensively but not exclusively as a source example). Early angiosperms appear mostly to have woods suited for moist localities without extremes of temperature or water availability. These woods are characterized by a high degree of conductive safety (i.e., disabling of the conductive system by air embolism is prevented). Angiosperms that have colonized more seasonal habitats ("breakouts") have xylem suited to rapid water flow, but subject to embolism formation, and these angiosperms have acquired mechanisms for embolism reversal. The features that make for safety (tracheids; short vessels, scalariform perforation plates in vessels) shift to alternative conditions that favor rapid flow (simple perforation plates, longer vessels). The groups with simple perforation plates and associated features have speciated rapidly, resulting in "breakouts" as seen on phylogenetic trees. Both "primitive" and "specialized" wood patterns are adaptive—but in different ecological settings. Knowledge in plant anatomy must be joined with understanding of molecular phylogenetics, ecology, systematics, cell microstructure, and wood physiology to create a new understanding of evolutionary patterns in wood.
One of the insights that came out of doing this paper was that wood evolution isn't a mater of shifting character states forward and backwards. Change in wood anatomy seems, with a few exceptions, to be progressive, with new changes built on top of older changes, and new and unexpected changes frequently occurring. This is probably one reason why a number of woods (but not all) can be identified. There aren't just a few plans operative, there are many. Different wood plans often coexist in trees of the same locality.
Another insight was that to shift to a new modality, a wood doesn't have to be radically changed. Thicker walls on libriform fibers can make it stronger. Making some imperforate tracheary elements near vessels have bordered pits (= vasicentric tracheids), other elements farther away have simple pits and are thus libriform fibers (the wood of Quercus for example) involves what seems to be a simple change. Oak wood makes bordered pits on vessel elements (and oaks were probably derived from rosids with tracheids as a background cell type. Nearly all vascular plants have genetic information for making simple pits as well as information for making bordered pits, so its a matter of differential application of material already on hand, not invention of a new cell type. Likewise, the dimorphisms (see fiber dimorphism, tracheid dimorphism, and vessel dimorphism elsewhere in the Wood Evolution section) are examples of just such changes.