The roots of a tree serve to anchor it to the ground and gather water and nutrients to transfer to all parts of the tree, and for reproduction defense, survival, energy storage and many other purposes. The first root produced by a newly germinated seedling is a taproot which goes straight downwards. Within a few weeks lateral roots branch out of the side of this and grow horizontally through the upper layers of the soil. In most trees, the tap root eventually withers away and the wide-spreading laterals remain. Near the tip of the finer roots are single cell root hairs.
In the soil, the roots encounter the hyphae of fungi. Many of these are known as mycorrhiza and form a mutualistic relationship with the tree roots. Some are specific to a single tree species, which will not flourish in the absence of its mycorrhizal associate. Others are generalists and associate with many species. The tree acquires minerals such as phosphorus from the fungus while it obtains the carbohydrate products of photosynthesis from the tree.
Some trees such as the alders (Alnus spp.) have a symbiotic relationship with Frankia sp,, a filamentous bacterium that can fix nitrogen from the air, converting it into ammonia. They have actinorhizal root nodules on their roots in which the bacteria live. This process enables the tree to live in low nitrogen habitats where they would otherwise be unable to thrive. Researchers have discovered that certain plant hormones called cytokinins initiate root nodule formation and that this process is closely related to the mechanisms involved in mycorrhizal association.
The long cold winter is unsuitable for plant growth and trees must grow rapidly in the short summer season when the temperature rises and the days are long.
It has been demonstrated that some trees are interconnected through their root system, forming a colony. The interconnections are made by the inosculation process, a kind of natural grafting or welding of vegetal tissues. The tests to demonstrate this networking are performed injecting chemicals, sometimes radioactive, in a tree, and then checking for its presence in neighbor trees.
In the case of Angiosperms and Gymnosperms, the outermost layer of the trunk is the bark and is mostly composed of dead cells. It provides a thick, waterproof covering to the living inner tissue. It protects the trunk against the elements, disease, animal attack and fire. It is perforated by a large number of fine breathing pores called lenticels through which oxygen diffuses. Bark is continually replaced by a living layer of cells called the cork cambium.
The tree reacts to the growth of the fungus by blocking off the xylem tissue carrying sap upwards and the branch above, and eventually the whole tree, is deprived of nourishment and dies.
Although the bark functions as a protective barrier, it is itself attacked by boring insects such as beetles. These lay their eggs in crevices and the larvae chew their way through the cellulose tissues leaving a gallery of tunnels. This may allow fungal spores to gain admittance and attack the tree. Dutch elm disease is caused by a fungus (Ophiostoma sp.) carried from one elm tree to another by various beetles.
Buds and growth
Trees do not usually grow continuously throughout the year but mostly have spurts of active expansion followed by periods of rest. This pattern of growth is related to the climatic conditions, growth normally ceasing when conditions are either too cold or too dry. In readiness for the inactive period, trees form buds to protect the meristem, the zone of active growth. Before the period of dormancy, the last few leaves produced at the tip of a twig form scales.
When growing conditions improve, such as the arrival of warmer weather and the longer days associated with spring in temperate regions, growth starts again. The expanding shoot pushes its way out, shedding the scales in the process.
These are thick, small and closely wrapped and enclose the growing point in a waterproof sheath. Inside this bud there is a rudimentary stalk and neatly folded miniature leaves, ready to expand when the next growing season arrives. Buds also form in the axils of the leaves ready to produce new side shoots. A few trees, such as the eucalyptus, have “naked buds” with no protective scales and some conifers, such as the Lawson’s cypress, have no buds but instead have little pockets of meristem concealed among the scale-like leaves.