Research
1. What is wood and what is different about the wood of tulip tree?
The account that follows is based on a paper in the journal New Phytologist, published on 30th July 2024: https://doi.org/10.1111/nph.19983
As every schoolboy knows, plant cells are surrounded by a cell wall. Cellulose is the principal component of these walls. Cellulose molecules are long, thin and have tensile strength. One molecule’s strength is limited, but cellulose molecules are cross-linked into groups known as microfibrils which are 3-4 nanometres in diameter. Microfibrils are glued together into macrofibrils by a complex carbohydrate called hemicellulose. There are many macrofibrils in a cell wall (visible in the electron micrograph below). Plant cells are typically pumped up to a high pressure like a bicycle tyre, but the strength of all the cellulose molecules in the cell wall prevents bursting.
When plants were becoming adapted to life on land during the Silurian period (430 or so million years ago) there several challenges, two of which were these:
* supporting aerial parts of the plant against gravity
* transporting water up to the aerial parts.
Both of these challenges were met by the evolution of xylem tissue with thickened cell walls composed of macrofibrils impregnated with lignin. These walls are very strong, so xylem can transport water under suction without imploding. Xylem also provides the support needed to resist gravity and avoid damage due to wind. Xylem is present in all conifers and flowering plants, but trees produce particularly large amounts. This is the wood that forms the centre of trunks and branches.
The diameters of cell wall macrofibrils in many different tree species have been measured in a recent research programme. A clear overall trend was found: gymnosperms such as pines, redwoods and cypresses have wider macrofibrils than angiosperms (flowering plants) such as oaks, limes and eucalypts. The average diameters were 26 to 29 nanometers versus 16 to 18 nanometers. However, trees in the genus Liriodendron were exceptional, with macrofibrils that were larger than all other angiosperms and smaller than those of gymnosperms. The average diameter for Liriodendron tulipifera was 22.4 nm and for Liriodendron tulipifera was 20.7 nm.
Liriodendrons diverged from other flowering plants 30 to 50 million years ago at a time when atmospheric carbon dioxide concentrations declined from 1000 to 500 ppm. Liriodendron trees are very efficient absorbers of carbon dioxide from the atmosphere. They store large amounts of carbon in compounds such as cellulose in the enlarged macrofibrils of xylem cell walls. Liriodendrons are therefore potentially very useful for carbon sequestration, so should be more widely planted. Liriodendron tulipifera grows to more than 55 metres tall and 3 metres in diameter in its native range from New England to Florida.
AJA © 16th September 2024