Science

  • Issue 108 / November - December 2015



    The Design of the Vascular Tissue in Plants

    Ali Erkan Uguz

    Just as humans have blood vessels under their skin, leaves also have vessels. These vessels transport water and various nutrients around through the leaves and trunk of the plant. Be it giant sequoias reaching to the heavens, or smaller plants like pines or apple trees, water and nutrients are carried through these veins to cells.

    In humans and many animals, nutrients are transported via the rhythmic beats of the heart, which acts like a pump. In plants, no organ is present to pump water and nutrients. Despite this, nutrients are transported non-stop through their bodies. How?

    This movement is enabled via perfectly planned biochemical and physical laws. Nutrients that are obtained from the environment or synthesized in cells are transported in their own vascular channels. Minerals, vitamins, fructose, and hormones are delivered by these vessels to cells, sometimes hundreds of meters away.

    The different parts of trees have different mechanisms for keeping them healthy. The rigid, long, powerful support tissue (sclerenchyma) and the ground vascular capillaries (parenchyma) assist with the support and transport of materials in the trunk. Thick and meter wide trunks, made of hard, lengthy fibers can withstand winds and storms for thousands of years due to their special architecture, which delivers organic materials to the entire organism.

    The process begins in the roots, which have very critical tasks, as well as morphological and physiological specifications. The root tip advances deep into the soil, using its regenerative, cone-shaped, protective tissue (the calyptra). Damaged and lost cells at the tip are replaced. The upper tissue layer has absorbent hairs (Epidermic cells) that take in water and minerals from the soil after differentiating according to their genetic program.

    After being absorbed through the roots, nutrients and water are taken into the plant's "vessels." These vessels are lifeless ligneous ducts that can quickly transport water and many minerals via long and sturdy channels (xylem). Living channels with filter-like porous walls (phloem) slowly transport organic materials to the necessary tissues. In these porous cells, the nuclei and some membranous organelles are eliminated to facilitate material transport.

    Nearby companion cells help support metabolism. This kind of cooperation and communication are routinely observed in the natural world, showing an incredible compassion between cells and organisms through electromagnetic, ionic, and nuclear forces of molecules.

    The xylem and phloem vessels feature the finest forms of the arts of endurance, decoration, distribution, architecture and design, and they are produced from reproductive and differentiating cells (called Meristem tissue). They exist in tree trunks that are hundreds of meters high, and also in tiny ferns. The roots of perennial plants are as robust as the columns holding up a sea platform. This architectural feature helps support the plant's body in the best way.

    Another part of the process, and one of the most important features of plants, is the synthesis of organic food material with the help of sun rays and photosynthesis. The transport of nutrients generated via photosynthesis inside the vascular tissue is fascinating. Elements of the vascular tissue carry out different tasks; cooperation is once again key: the porous channels carry organic material and the ligneous tubes carry water. Vascular bundles transport these nutrients from one leaf towards the root cell via diffusion, active transport, and fluidic pressure. Bark, on the outer part of a tree's trunk, is merely protective ÔÇô much as skin is for humans.

    There are assimilation cells in charge of photosynthesis in plant leaves. Traveling on these cells, water and solute material transit towards the major vascular bundles via cytoplasmic (the symplast) or cell wall channels (the apoplast). Conversely, the cells providing nutrients to photosynthetic, organic food synthesizing cells and demanding tissues, are source cells. By utilizing carbon dioxide, water, or nitrous salts together with energy coming from light, various foods are produced in the source cells. With the help of many chloroplast organelles, as well as the chlorophyll and enzymes inside the source cells, the organic materials which have been produced are conducted to companion cells. These nutrients pass into sieved, porous channels from the companion cells.

    As nutrients pass into the semi-empty, living, porous parts of the cell walls, their fluid absorbing capacity also improves. By releasing some water from the neighboring lifeless, ligneous channel bundles, water pressure forms in the porous cells. The nutrient flow is maintained at a stable and sized speed thanks to the finest architecture of and rigidity found in the system.

    Fructose, sucrose and other important nutrients easily pass towards the tissue cells from the porous cells as the fluid pressure grows. When necessary, the right amount of food is stored in preparation for winter or harsh weather.

    The last part of the process enables the transfer of nutrients. Through active transport, diffusion, and pressure flow, the required organic nutrients can be transferred everywhere. The cells that enable the transfer are the pool cells. They generate a great osmotic pressure density at the roots.

    During the processing of food, possible harmful substances like mud, or carbon dioxide taken from the air, are processed and converted into wonderful nutrients. Some of these nutrients are even converted into food for people and animals. In the pool cells, many delicious fruits like pomegranates, oranges, grapes, and cherries are produced. While plants consume the mud and carbon dioxide themselves, they offer the best of food to humans and animals in a beautiful program of art and creation.

    The whole process is really quite miraculous when you look at the entire things:

    Water and dissolved minerals are received from the soil by the absorption of the epidermis cells. By transpiration, the pull between the hydrogen atoms of the water molecules (cohesion) enables the transport of liquids in the ligneous tubes all the way to leaf tips. Water molecules in the capillary shaped ligneous tubes rise quickly, with a physical force. Water and salts obtained from the soil are, in a way, pumped to all organs with the assistance of the fluidic osmotic pressure in the roots. The transport of water to higher levels is better facilitated by a different attraction force (adhesion) between the vascular bundles and water molecules. In time, the pool cells take in and store organic nutrients with the help of their receptor structures, thus lowering the density of the porous channels. Due to the osmotic balance principle, the excess waters are returned back to the ligneous tubes. The material transport speeds up during the day because of transpiration and photosynthesis, and it slows down during the night.

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