Luminous wood

On the images you can see a cross section of a thin stem of Tilia showing the annual rings which are composed of Xylem in botanical terms or wood in daily language.

At the beginning of the growing season in a temperate climate, the Cambium will produce larger Xylem cells with thinner walls. These large cells (earlywood) are formed because the tree needs a lot of sap after winter. The earlywood is lighter and more porous. At the end of the growing season, the Cambium will produce narrower cells with thicker walls (latewood). Thus latewood contains more matter and is less porous, so it will look darker. Earlywood and latewood, which are formed during the same growing season, make up one annual ring. The most recent growth rings are situated at the tree's periphery, under the Cambium which is between the wood and the bark.

 

Xylem is a specialized tissue of vascular plants with several functions: it transports water and nutrients from the plant–soil interface to stems and leaves; provides mechanical support and posture control and its involved-on reserve storage and defense processes.

The peripheral section of the Xylem, the sapwood, conducts crude sap. This contains amongst other things minerals and flows upwards from the roots to the leaves. The elaborated sap containing sugars circulates downwards to the roots in the Phloem, or the inner bark. On the fluorescence image with the biggest magnification, bordered pits can be seen in the cell walls. These are responsible for the exchange of fluids between the cells.

On the transverse section shown here, the Medullary Rays as lines oriented toward the center of the tree, the pith. In a Xylem cell wall, there are hydrophilic elements which have an affinity for water: cellulose and hemicelluloses, lignin is another component.

Certain woods can appear basically identical to one another under normal lighting conditions. When exposed to UV light, the Xylem will absorb and emit light in a different (visible) wavelength. This phenomenon is known as fluorescence, and certain woods can be distinguished by the presence or absence of their fluorescent qualities.

One of the best examples of fluorescence is found in Black Locust (Robinia pseudoacacia), which is very similar to Mulberry (Morus spp.) in both appearance and weight. But one way to easily distinguish the two is by observing them under UV light; Black Locust will emit a strong yellow-green glow, while Mulberry will be non-reactive.

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