BARK
The bark is an important part of a tree, protecting the vital growing layers of cells below from varying environmental conditions. It is produced by a layer of cambium cells and grows to accommodate the increasing girth of the tree. It may be thin and papery, smooth and shiny, or thick and deeply furrowed. Each type of bark is matched to the tree’s environment, so tree species that are subject to heat and strong sunlight in their native ranges have a thicker bark than those that come from cooler, humid climates (see also pp.).
Himalayan Birch bark is relatively thin and peels readily.
while Pyrenean Oak bark is thick and corky.
ROOTS
The first part of a tree to emerge from a seed is a tiny root whose first function is to draw up water and dissolved minerals from the soil. In the case of most of our tree species, successful germination is dependent upon this first root making contact with a species-specific symbiotic fungal partner, a relationship that continues for the rest of the tree’s life. This relationship is discussed in more detail on p. 34. From this simple start the tiny root will grow and divide, eventually forming a large network of powerful roots, side-branches and fine root hairs that spread out in all directions around the base of the trunk. The main roots will be woody and very strong, but their many branches terminate in fine root hairs that are only a few cells long; these have thin, permeable walls through which will pass all the water and minerals needed for the survival and growth of the tree. Although the sturdy roots strengthened with woody tissue help anchor the tree in the ground, it is the millions of fine root hairs that keep the tree alive by supplying it with water and nutrients. These fine root hairs are very short-lived, being constantly replaced as the main roots grow through the soil.
The root system of a large, mature tree does not penetrate far down into the soil. The most useful supply of dissolved nutrients for the tree lies in the shallow layer of topsoil and the adjacent sub-soil, so it is more beneficial if the roots spread outwards through this layer rather than penetrate to a great depth into a rather sterile and hostile layer. A 50mtall tree will probably have a spread of smaller branching roots all around the bole, the extent approximately equal to the spread of the branches or, sometimes, to the height of the tree. The proximity of other trees, the nature of the soil, and the presence of obstacles like rocks or river banks will all influence the final extent of the root system, however. This knowledge of the spread of the roots is useful when planning where to plant large trees that may damage drains or the foundations of buildings when they reach maturity, and it should also be borne in mind when digging ditches or ponds near large trees.
Spreading Beech roots.
In order to be able to function at all, roots require a supply of nutrients from the leaves, so within the root system there is a two-way traffic of water and minerals up from the soil to the leaves, and dissolved sugars and other nutrients down from the leaves.
The root hairs are living cells that require oxygen in order to be able to carry out respiration. They give off carbon dioxide as a waste gas, so they need access to air in the soil to allow these gases to circulate. Most trees, and most land plants, cannot grow in completely waterlogged soils and those that do have special adaptations for survival.
A number of trees, especially members of the Fabaceae, such as the Honey Locust, have many rounded nodules on the roots that contain colonies of nitrogen-fixing bacteria. These are able to use gaseous nitrogen and turn it into compounds vital to the growth of the plants.
LEAVES
Leaves are among the most conspicuous and distinctive features of any tree. They grow in a huge variety of shapes, sizes, colours and combinations and are usually the best feature for identifying the tree because of their unique structure. Leaves may vary from one species to another but they all perform the same vital function as the principal producers of food for the tree.
The first pair of leaves to emerge from the seed are simple, and bear no resemblance to the true leaves of the tree; they are derived from the seed’s food store. They are green, however, and supply the tiny seedling with its first food made from sunlight energy. Once the seedling has begun to produce leaves that are miniature versions of its true leaves, growth can begin very rapidly. Tiny seedlings are vulnerable to grazing, trampling, drought and competition, so very few survive.
Recently germinated oak seedling.
A plant’s leaves are its powerhouse, trapping energy from sunlight and converting it into basic food.
Leaves are basically thin layers of living tissue with the ability to trap light energy and use this to convert the raw materials of water and carbon dioxide into a simple sugar. This reaction, known as photosynthesis, is arguably the most important chemical reaction in the world, for it is the basis of all other food production. Animals do not have the ability to convert these simple materials into food; they have to rely on plants to do it for them. The simple sugar produced in the leaves is glucose, and this can be formed into a variety of other important materials, particularly starch, which many plants store, or pack into their seeds. A vital by-product of this reaction is oxygen, the gas essential for the respiration of all members of the animal kingdom. This explains the vital role of trees in the ecosystem: they are major consumers of carbon dioxide, one of the so-called ‘greenhouse gases’; and they are major producers of oxygen, the gas we need for our respiration. They are also major producers of food for much of the animal kingdom.
Contained within a leaf are numerous specialised cells. Some are concerned with the transport of materials in and out of the leaf, some are the vital energy-trapping cells that utilise sunlight, and others are concerned with the regulation of water movements. The cells that trap light-energy contain a light-absorbing pigment called chlorophyll, which gives leaves their green colour. Other pigments of different colours may be present in varying amounts, and it is this variety that gives leaves of different trees their own subtle shade of green. Without the green chlorophyll or other light-absorbing pigments, leaves would be unable to perform their important function, and also, if deprived of light, they would be unable to manufacture the tree’s food.
Leaves arrange themselves in such a way to absorb the maximum amount of sunlight, so spreading canopies or trees growing taller than their neighbours, are both ways in which trees maximise the light-gathering power of their leaves. Some leaves have paler patches that lack green chlorophyll; these are known as variegated leaves and certain trees, such as some cultivars of the Highclere hollies, regularly produce green-and-yellow leaves. If the leaves were completely lacking in chlorophyll they would be unable to manufacture food for the tree; the small areas of green tissue in the leaf produce all the food needed by the whole leaf.
All leaves have tiny pores in their surface (normally just the lower surface) called stomata. These allow water to evaporate into the atmosphere. To some extent the tree can regulate the opening and closing of these stomata, but during daylight hours, when the tree is trapping sunlight, they will be open, allowing water out and also allowing the circulation of the gases involved in photosynthesis. This can lead to problems for trees growing in hot, dry areas, or in well-drained soils where little ground water is available. In order to allow the essential gases to circulate, and at the same time minimising