Wednesday, March 8, 2017

The nearly forgotten art of comparative plant anatomy 1. Wood

The study of human anatomy is an obviously essential subject for all medical students, and most of us  have a basic idea of where our liver, lungs, large intestine, etc. are located - important information for identifying and fixing health problems.

Comparative anatomy, on the other hand, is about the differences and relationships among different organisms.  Most of what we know about dinosaurs comes from their bones.  Dinosaurs with bones of similar shape and arrangement are considered closely related to one another, and so we can reconstruct not only individual dinosaurs from their bones, but also phylogenies (family trees) and classifications.  Bones, by certain structural characteristics can also give us clues about what dinosaurs ate and whether they were warm-blooded or cold-blooded.

The comparative anatomy of plants has been put to similar use, helping us to interpret fossils, as well as to help identify and classify living plants.  Wood anatomists can identify the genus, and often the species, of a particular wood sample, and this is put to use by archaeologists and forensic scientists as well. How often has Abby from the hit TV show NCIS led her colleagues to a crime scene in the woods, by identifying wood, leaf, or pollen fragments from a murder victim's body or clothing?  Wood scientists, like those at the USDA's Center for Wood Anatomy Research, maintain a library of wood samples, including thin sections for microscopic examination.

The academic study of plant anatomy has, however, been in decline for decades (see Dengler 2002) .  This is partly because of the decline in botany courses in general, but also because of the need to make room in biology curricula for the massive amount of new material in ecology, cell biology, genetics, DNA sequencing, genomics, etc.  Plant anatomy in particular is considered by many to be old-fashioned, having largely been "done" in the 19th and early 20th centuries.  The study of plant anatomy, traditionally taught by looking through microscopes at prepared slides and making drawings, is admittedly tedious, requiring an interest and passion that is rare among most modern students.  To be of any use, one must acquire a broad knowledge of the different cell types and tissues of plants, and to recognize them as they vary in form from one species to another.  So it is a type of training, involving "memorization" of terminology and structural detail that is incompatible with the emphases on principles, theory, and experimental methodology in modern biology courses.  Moreover, the specialized equipment and skilled personnel for making thin microscope slides is rarely found anymore in biology departments.

Yet there is so much that we can learn still by comparing the anatomical structure of different kinds of plants.   Aside from practical applications in forestry, forensics and archaeology, it is still important in understanding the relationships and adaptations of plants in general.  The next few postings will illustrate some of this.

Let's start with the obvious: the structure of wood.  We will note first that plants are simpler that animals, and in plant anatomy, we're talking more about tissues than organs, actually more about what they would call histology in animal studies.  A tree trunk, composed of wood and other tissues is a single organ, so it is the different layers of tissues and cell types that determine how it functions, how it is adapted for its environment, and how one species differs from another.

True wood, as found in gymnosperm and dicotyledonous trees, consists of layers of xylem tissue added each year that increase the thickness of the trunk, branches, and roots.  Each layer is made up of water-conducting cells, and various sorts of supportive cells ranging from thin-walled parenchyma to thick-walled fibers.

The wood of  balsa (Ochroma pyramidale) is soft and light because the supportive
tissue around the large vessels is thin-walled.
 Photo from  Curtis, Lersten, and Nowak, University of Wisconsin, Stevens Point.

As an extreme example, the wood of the balsa tree is radically different from from that of a
teak tree, allowing us to not only recognize each under the microscope, but also telling us something about their different life styles.  Balsa is a fast-growing, short-lived tree that colonizes disturbed areas in forests and then gives way to longer-lived trees with more durable wood like mahogany or teak.  The density of wood is determined primarily by the abundance and distribution of fibers around the thinner-walled water conducting cell.  Durability is also enhanced by preservative chemicals secreted into the wood.
Among the great variety of dicotyledonous trees, there is great variety in the shape, arrangement, and abundance of these different kinds of cells. This variation has resulted in some woods being superior for fine cabinetry, others for resilient baseball bats (ash), durable bowling pins (hard rock maple), or strong shovel handles (hickory or ash). The wood database contains extensive images and data concerning the characteristics and uses of different kinds of wood.

In the tropical hardwood teak (Tectona grandis), the supporting tissue around the large vessels is made up mostly 
of narrow, thick-walled fibers.

In temperate trees, such as this red maple (Acer rubrum) the annual growth rings of wood
(horizontal bands) are marked by distinct boundary layer of smaller, thick-walled cells laid down in the Fall,
something you don't see in tropical woods.  The vertical bands of cells are made up of living
parenchyma cells, often with thick secondary walls.
The flexible woody stems of grape vine (Vitis spp.) contain exceptionally wide
rays of parenchyma tissue that separate the narrow wedges of vessels and other
supportive tissues.
The simpler, softer wood of conifers, like pine or fir, consists mostlyof a single type of relatively narrow cell, the tracheid, which combines strength with water-conducting ability.  As angiosperm trees diversified, ancient tracheids diverged evolutionarily into two kinds of cells. Strength and density functions were taken on as some cells developed thicker walls, becoming fibers.  Water-conduction was focused in other cells that remained thin-walled, but became wider, and shorter: the  stacked cylindrical cells of the vessels.  There is also a network of parenchyma cells that permeate the wood as flat rays or vertical strands
The wood of conifers in general is very simple, consisting mostly of  tracheids, which conduct water as well as provide
structural support for the tree.  The band of narrower, thick-walled cells running across the bottom of the image is a
boundary layer, marking the slowing and  cessation of growth in the Fall.  Above it, the uniform mass of cells produced
in the Spring and Summer.  

In pines and some other conifers, the uniform growth of
cells is interrupted by large resin canals.

In monocots, the ability to produce layers of wood was abandoned as their ancestors evolved underground stems and clonal lifestyles.  When some of them became trees again, as in palms or bamboos, their water-conducting cells remained in separate vascular bundles scattered throughout the interior of the stem and surrounded by dense sheaths of fibers.
The dense, wood-like property of a palm trunk is provided by dense masses
of fibers, not layers of xylem laid down in rings. In this preparation, the walls
of the fibers are stained green rather than the usual red.
The rigid wall of a hollow bamboo stem is filled with  fibrous bundles.

On-line wood anatomy resources:

Curtis, Lersten, and Nowak, Photographic Atlas of Plant Anatomy, revised 2015

Schoch,W., Heller,I., Schweingruber,F.H., Kienast,F., 2004:  Wood anatomy of central European Species.

Meier, Eric,  The Wood Database.

Inside Wood, North Carolina State University, developed by Elizabeth Wheeler and others.  This site is more technical, and contains the most extensive library of wood anatomy images.

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