Trees: the giants of the plant world

Trees are the largest in the kingdom of plants, even larger than the biggest animals in the world. Mind you: an adult female Blue Whale measures “only” 33 meters in length and weighs a “mere” 190 tons. The secret of trees lies in their unique structure. Let’s have a closer look at it.
Trees have a woody stem, or trunk, formed by secondary growth during the entire life of the plant. It means that the trunk thickens each year by growing outwards, in addition to the primary upwards growth from the growing tip. Wood also gives structural strength to the trunk; supporting the plant as it grows larger. This structure and rigidity is mainly due to lignin, an aromatic polymer, depositing in the cell walls of terrestrial plants.
The cells from which all plants grow are called meristem. They are found at the tips of buds and roots. In most woody plants, there is also another layer of reproductive cells called the cambium.
To explain it in detail, inside a living tree trunk, there are several layers of cells. The outer layer is the bark. Bark is tough and waterproof and protects the tree from the elements, insects, pests and fungal diseases. Bark also helps the tree to retain its moisture. As the trunk grows fatter, the bark spreads and cracks, often becoming gnarled in appearance.

Underneath the bark, is the phloem. This transports sugary sap made during photosynthesis from the leaves to all the other parts of the tree.
The next layer is the cambium, a reproductive layer of undifferentiated cells that are continually dividing. It allows trees to grow outwards as well as upwards in each growing season, creating phloem cells on the outside and wood cells, called xylem, on the inside. These new cells are aligned in growth rings, which are the increment of wood added during a single growth period. In temperate regions where the growth period is usually one year, by counting these “annual rings” we can tell the age of the tree.
There are several layers of xylem beneath the cambium. The first few layers are known as sapwood. They are responsible for transporting water from the roots to the rest of the tree. The innermost layers of xylem, right in the centre of the tree, are known as heartwood. The heartwood is dense and strong and provides stability as the tree grows. It is made up of clogged secondary xylem that is dead cells where special chemical compounds such as resins, terpenes, and phenols accumulate. These compounds give the heartwood a darker colour and also offer resistance to decomposition and the ability to prevent insects and fungal infections. Trees with nice coloured durable heartwood – for instance oak, cherries, walnut, black locust, and many exotic tropical species like Indian rosewood and African rosewood – are especially favoured for furniture making. Hardwood species (beech, walnut, Black locust, oak, ash, hornbeam etc.), which are generally more valuable for the timber industry, contain more lignin whereas softwood trees (spruce, Scots pine, lime, alder, poplar etc.) contain less lignin and more cellulose.

Due to their special structure, trees can reach unbelievable heights. The tallest of all is the Coastal Redwood (Sequoia sempervirens) in California, which can grow to over 100 metres in height. It just shortly beats the Australian Mountain Ash (Eucalyptus regnans), ranked second. Some lianas can boast with an even longer woody stem, like certain Calamus species whose creeping stem can reach a length of 180 metres. It is used for making popular rattan furniture. By volume, the largest trees, and living things for that matter, on Earth today are the Giant Sequoias (Sequoiadendron giganteum). They can weigh up to 2,500 tons.
Trees excel in another field too: longevity. The oldest specimens, some of them 4,700 years old, belong to the species of Rocky Mountain Bristlecone Pine (Pinus aristata). They are native to Colorado and northern New Mexico.
In Europe we do not have such long-lived trees. Our oldest ones come from limes and oaks, reaching an age of 1,000-2,000 years. We should protect them, respect their age. The ancient “wise trees” in our historic gardens have already witnessed centuries of history and they can see a lot more if taken good care of. Let our great-grandchildren admire them many years from now.

Erzsébet Fráter