You Cannot See the Wood for the Trees

Surely you wood know

David Beeson

Sitting under a walnut or apple tree when the fruit is ripe is hazardous. A chunky apple or a dozen woody nuts may aim for your head. Ouch! A half-kilogram apple would certainly hurt me. Yet, the tree holds dozens of them aloft … and all the branches, leaves, a dozen squirrels and thirty rooks. How does it manage it and why?

Why? That’s easy. Any plant’s chemistry, its metabolism, is driven by energy – sunlight energy trapped in photosynthesis. The leaf’s internal organelles, called chloroplasts (the only green bits), absorb light energy (photons) and use their energy to bind water and carbon dioxide together into energy-rich fats and, especially, carbohydrates. Leaves are energy transfer machines, a bit like a photo-electric panel on the roof.

Within reason, the more incident light the more energy is available. Once trapped the energy can be used to drive the plant’s growth and reproduction.

Plant leaf structure. Cutaway computer illustration showing the structure of a typical plant leaf. In most plants, leaves are the major site of photosynthesis. Chloroplasts within the leave’s cells use chlorophyll to convert the energy in sunlight into chemical energy that the plant can use as food in the form of glucose. Each leaf is made of many layers sandwiched between two layers of tough epidermal skin cells, which also secrete a waxy substance that forms the cuticle. These layers protect the leaf from pests. Within the epidermal cells are pairs of guard cells, which form a pore (stoma), the site of gas exchange. Most food production takes place in the palisade mesophyll. Gas exchange occurs in the air spaces between the cells of the spongy

A strong woody stem can be employed to lift that plant’s leaves above the competition. They will be able to outcompete non-woody plants, like primroses or daffodils. Unfortunately, wood is expensive for the plant to manufacture – so the woody plants are often comparatively slow growing.

Wood grows from living cells – the cambium meristem, located between the water-carrying xylem (wood) and the food-carrying phloem (towards the outside of a stem or trunk).

Xylem is composed of long cells that are joined end to end. Their cellulose cell walls are slowly impregnated with a chemical called lignin (waterproofing and a preservative) that changes cellulose into wood. In the process the cells are killed. Wood is dead.

Woody stem. Xylem is the WOOD. This stem is just three years old. This is a cross-section.

There are two main components to xylem. One is mainly found in conifers the other mainly in the non-coniferous plants. The non-coniferous system is more efficient.

As you’ll know already, but I will recap, it is heat that (mainly) drives the water flow up the xylem. Water molecules are evaporated (Evapo-transpiration) by heat from leaves and that pulls more water up the stem, main roots and in via the minute root hairs on the smallest roots. Dissolved in the water will be the minerals required for the metabolism of living cells.

Evapo-transpiration is not the only way water is driven through the plant. In some, but not all cases, the roots can pump water upwards. This is VERY expensive for the plant and is really a last resort. But plants do use it in the spring when the leaves are not present or still growing. You may have noticed this if you cut a leaf-free stem or trunk and it bleeds.

Xylem pipes are plumbed into the leaves and flowers. In autumn that section of xylem plumbing becomes mostly redundant, and only the wood towards the outside of a trunk carries water as it is connected to new growth. So, a hollow trunk is not a big disadvantage to a tree. In fact, a hollow trunk can be stronger than a solid one and, if fungi breakdown the wood, the nutrients can be absorbed and reused by the tree. A double win.

Because wood is expensive plants try to avoid making much of it. Primroses and daffodils contain very little and hope that they can keep out of the shade; they stay small and quick growing. Clematis plants, like old man’s beard, save wood by climbing over woody plants and still reach the full sunlight.

Wood is fascinating stuff! Builders have used it since the ‘dawn of humans’, it gets trees and shrubs up into the light and it holds apples high enough to fall on Sir Isaac Newton’s head and give us the theory of gravity.

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