In my 1958 paper on wood anatomy of Heliantheae, I described a phenomenon that I termed fiber dimorphism. I noticed this particularly in Dubautia, in which there are bands of thinner-walled fibers at intervals within a background of thicker-walled fibers. The thinner-walled fibers are also shorter and are storied, whereas the thicker-walled fibers are longer and nonstoried. The difference between the two is less in Dubautia laxa. In a transection of the wood of D. laxa, the two types of fibers aren’t conspicuous: the areas of thin-walled fibers do look paler, as they are in the tangential section also. In a transection of the wood of D. menziesii, the patches of thin-walled fibers are more easily seen. The difference between the two types of fibers is considerable in D. sherffiana. Dubautia has axial parenchyma, so the wider, thinner-walled fibers must represent not a form of parenchyma, but a division of labor in libriform fibers: storage in thin-walled cells, with greater mechanical strength in thicker-walled cells? That’s a working hypothesis. One can see fiber dimorphism in the two other Hawaiian genera of the silversword complex, Argyroxiphium and Wilkesia.
Not just in Hawaiian composites. In Acer, Albizia, and Fraxinus, living fibers sheathe vessels, whereas nonliving fibers occur farther away from vessels. The living fibers in the sugar maple, Acer saccharum, contain starch which changes to sugar at the end of winter, increasing the sugar content of vessels, which thereby pull sap up into the tree from the roots, which before the growing season are not absorbing water very actively. So fiber dimorphism accounts for maple syrup. Other genera in which fiber dimorphism occurs include Capparis (Brassicaceae s.l., or Capparaceae), Triplaris (Polygonaceae), and Allophyllus and Paranephelium of the Sapindaceae. The functional explanation in most cases probably is a division of labor between fibers which serve for photosynthate storage and fibers which serve for mechanical strength. One should not include in this phenomenon instances in which latewood fibers are thicker-walled, as in Averrhoa (Oxalidaceae).
Now the interesting thing to me is the fact that this phenomenon hasn’t received mention in wood anatomical literature of the last several decades. Fiber dimorphism may not occur in wood of a large number of plants, but some of the plants in which it does occur, such as sugar maples, may be common. But more importantly, it was for me an opening into fascinating and unappreciated evolutionary phenomena that woods show. If something like fiber dimorphism was occurring, what else that hadn’t be in woods might be found? The complexity of wood anatomy tends to make some workers want to study it in terms of simple definitions, but wood evolution is a flexible and nuanced process that escapes simple definitions: there are many pathways in wood evolution. I later discovered and documented dimorphism in fiber-tracheids, tracheids, and even vessel elements. Together, these instances form a fascinating story of how the ability to form a particular cell type can be modified in the direction of division of labor. The structure of wood should be an entry to understanding evolution and function, and how those occur in woody plants: if different woods occur in different plants. Describing the end-products is desirable, but understanding what led to the diverse types of wood is surely even more important.
Sometimes, new data come along and one sees an old discovery in a new light. That happened with the phenomenon of fiber dimorphism, and in 2013, I published a paper in which I tried to consider all conceivable types of fiber dimorphism—things like different diameters of fibers in latewood and earlywood, gelatinous fibers vs. non-gelatinous fibers, etc. But the main examples were from woody families in which narrow, long, thick-walled fibers occur in some parts of the wood, whereas wider, living (septate mostly), thinner-walled fibers occur in other parts. Sometimes they look similar, and these cases had mostly been overlooked. But the idea of a plant redesigning fibers so as to have two kinds instead of one was, I thought, a fascinating example of how two functions could be served, and the quantities and placement of the two different fiber types sensitively altered. Non-living fibers must serve most a mechanical function, but living fibers can store starch for flushes of growth, etc.—and correlations can be found. If we assume that changes in structure are adaptive (and aren't they?), then the anatomy of a wood is like a record of how that wood functions, and ultimately we should be able to read that record in terms of functions. Comparative anatomy is a story of how evolution works, and while interpreting that story fully may take time and new methods, it's a story that we can't overlook.