As the trunk grows in thickness, further changes in its structure take place. The outer parts of the wood, composed of active conductive elements and living cells, they can conduct water and store reserve substances. These pieces of wood are called white. The oldest (inside) while parts of the wood darken, the vessels become clogged with bubble-like formations, so-called inserts, resulting from the ingrowth of living cells into the vessels. These older pieces of wood are called heartwood; it mainly performs mechanical functions. In addition, due to the deposition of tannin derivatives in it, waxes, dyes, gums and resins, the heartwood is resistant to destructive external factors and attack by fungi and insects. Not all species have the same clearly visible sapwood and heartwood. For trees with typical heartwood (tough guys) belong: oak, cis, juniper, robinia, elm, nut, plum, larch, pine and others. Often in the heartwood of such trees, such as beech, inorganic impregnation substances are deposited, e.g.. calcium carbonate. Whereas spruce, fir, olsza, clone, horse chestnut, birch, hornbeam is considered to be the so-called sapwood, because they do not produce a colored heartwood. Linden trees, aspen and willow trees do not produce heartwood at all and therefore in old age the trunks of these trees are more easily infected, and their rotten interiors often form the so-called. "Chimneys". The upward transpiring current occurs exclusively from the outside, the whitewashed part of the young tree.
Drawing. A clipping of a four-year-old pine branch cut in winter (Pinus sp.); 1 - cork necrosis, 2 — łyko, 3 - cambium, 4 - late wood, 5 - early wood, 6 - annual increase limit, 7 - resin channel, 8 - core, 9 - primary spinal radius, 10 - secondary spinal ray, 11 - core rays in the tangential section, 12 - spinal ray in the mouthful.
Łyko (phloem) includes the following main ingredients (they have their counterparts in terms of origin and function in wood): screen tubes composed of members arranged longitudinally one after the other (sieve cells), with companion cells (conifers do not contain companion cells); bast crumb; bast fibers (sclerenchymatic) and secretory cells. In phloem sieve tubes, being the equivalent of wood vessels, there is conduction of assimilates mainly from the crown down the tree (downdraught). Unlike the dishes, members (cells) the sieve tubes are alive, albeit heavily modified, m.in. in their mature state they are devoid of a nucleus, and their protoplasts are joined by the so-called. sieve fields (clusters of pores in the end walls). Thickness of the active (conductive) phloem ring is equal to ash 0,2 mm, oak, Open, maple and birch 0.2-0.3, elm 0.4-0.7, willow and poplar 0.8-1.0 mm. It works for a relatively short time, e.g.. in conifers 1–2 years, in deciduous trees a few years, the longest time in linden - approx 10 years, and then, as a result of the changes taking place, the sieve elements cease to function. A much larger part of the non-conductive phloem contains viable parenchyma cells. The main role of this part of phloem is to store spare materials. The so-called substances also accumulate here. ergastyczne, usually phenolic compounds and crystals (mainly calcium carbonates and oxalates). As a result of normal growth of the crumb cells, the sieves are crushed, and the whole phloem grows and adapts to the growing circumference of the trunk.
Core rays run through the conductive and non-conductive phloem from the cambium, ensuring the transverse conduction of water and assimilates. The equivalent of wood fibers are sclerenchymatic phloem fibers. They are usually grouped into strands and give the bast a certain tensile strength (use of linden fiber).
Most of our trees produce cork-making pulp (fellogen). It is produced initially from the cells of the primary cortex, and then during growth in thickness - from the living cells of the phloem crumb. Fellogen produces a layer of cork on the outside (otherwise, cork or cortical necrosis on), and inside the felloderm - a layer of living cells containing chloroplasts, so green.
Together, we refer to all three layers as the cork (peryderma). Periderm occurs in plants characterized by secondary growth in thickness. Most often, the initial periderm is replaced by subsequent layers that form beneath it. Then there is a continuous formation of layers of cork necrosis. The cork gradually peels off, in particular species, different numbers of its layers remain on the trunk. Many layers of the periderm remain on the Scots pine (to several dozen), while on the trunk - just a few. The cork cells are filled with air, hence its lightness, flexibility and good insulating properties. The cork cell walls are saturated with suberin, waxes and tannins, therefore the cork is impermeable to water.
If the successively growing periderms are in the form of overlapping scaly layers, a scaly type of bark is formed, e.g.. in Scots pine. In some species, e.g.. in noise and hornbeam, The initial periderm may persist throughout the life of the plant, and in the oak for many years. In this case, the fellogen cells divide in the peripheral direction (anticlinical), that the circumference of this meristematic tissue may increase, from which the periderm is formed.
The main function of the bark is to protect the living, the delicate tissues of the trunk (phloem and cambium) against mechanical damage, thermal, chemicals and excessive water evaporation. At the same time, bark beetle must ensure the possibility of gas exchange of the trunk. The gases are exchanged by means of traces, i.e.. clearly separated parts of cork tissue with large intercellular spaces, or due to direct contact between the intercellular spaces in the living, deeper tissues - with the atmosphere. Most of the trees, for example, poplar, hazel, ash, produces more or less pronounced spiracles. In others, for example in Scots pine, spiracles are absent. In parts of the trunk covered with a thick layer of bark, like a pine tree, oak, larch and others, gas exchange takes place through the bark slots.
When tree trunks or branches are injured, cells adjacent to the wound are divided more quickly. This mainly affects the cambial cells.
As they grow larger, they produce healing tissue (callus), which gradually grows over the wound. Within the healing tissue, similar to the healthy part of the bast, all the processes that cause the cork to form are in progress. The healing tissue, after the wound surface is atgrown and in contact with the edges, heals and forms a uniform cambium ring. The phloem and wood formed in the healing tissue covering the wound then fuse together. However, the resulting wood does not fuse with the old wood under the healing tissue layer.
Drawing. Schematic cross-section of the wound edge and the healing tissue formed on a perennial branch, two years after the wound has developed; Km - cambium, Ds - old wood, Łs — łyko stare, Kr - kora, D1, D2 - wood of the first and second years, Ł1, Ł2 - bast of the first and second years, E - epidermis.