The appearance of conductive tissues in the process of evolution is one of the reasons that made possible the emergence of plants on land. In our article, we will consider the features of the structure and functioning of its elements - sieve tubes and vessels.
Features of conductive fabric
When the planet experienced major changes in climatic conditions, plants had to adapt to them. Before that, they all lived exclusively in the water. In the ground-air environment, it became necessary to extract water from the soil and transport it to all plant organs.
There are two types of conductive tissue, the elements of which are vessels and sieve tubes:
- Lub, or phloem - located closer to the surface of the stem. Through it, organic substances formed in the leaf during photosynthesis move towards the root.
- The second type of conductive tissue is called wood, or xylem. It provides an upward current: from the root to the leaves.
Plant sieve tubes
These are the conducting cells of the bast. They are separated from each othernumerous barriers. Outwardly, their structure resembles a sieve. That's where the name comes from. The sieve tubes of plants are alive. This is due to the weak downward current pressure.
Their transverse walls are pierced with a dense network of holes. And the cells contain many through holes. All of them are prokaryotes. This means that they do not have a decorated core.
The living elements of the cytoplasm of sieve tubes remain only for a certain time. The duration of this period varies widely - from 2 to 15 years. This indicator depends on the type of plant and the conditions of its growth. Sieve tubes transport water and organic substances synthesized during photosynthesis from leaves to roots.
Vessels
Unlike sieve tubes, these elements of conductive tissue are dead cells. Visually, they resemble tubes. Vessels have dense shells. On the inside, they form thickenings that look like rings or spirals.
Thanks to this structure, the vessels are able to perform their function. It consists in the movement of soil solutions of minerals from the root to the leaves.
Mechanism of soil nutrition
Thus, the movement of substances in opposite directions is simultaneously carried out in the plant. Botanically, this process is referred to as an upward and downward current.
But what forces make the water from the soil move upwards? It turns out that thisoccurs under the influence of root pressure and transpiration - the evaporation of water from the surface of the leaves.
For plants, this process is vital. The fact is that only in the soil are minerals, without which the development of tissues and organs will be impossible. So, nitrogen is necessary for the development of the root system. There is plenty of this element in the air - 75%. But plants are unable to fix atmospheric nitrogen, which is why mineral nutrition is so important to them.
Rising, water molecules tightly adhere to each other and to the walls of blood vessels. In this case, forces arise that can raise water to a decent height - up to 140 m. Such pressure causes soil solutions to penetrate through the root hairs into the bark, and further to the xylem vessels. On them, water rises to the stem. Further, under the action of transpiration, water enters the leaves.
Sieve tubes are located in the veins next to the vessels. These elements carry downward current. Under the influence of sunlight, the polysaccharide glucose is synthesized in the chloroplasts of the leaf. The plant uses this organic matter for growth and life processes.
So, the conductive tissue of the plant ensures the movement of aqueous solutions of organic and mineral substances throughout the plant. Its structural elements are vessels and sieve tubes.
