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Flowering Plants have a system of vessels that runs up and down the plants carrying materials known as transport or vascular tissues.
There are 2 types of transport tissues in plants: the xylem ( aka wood ) and the phloem.
overview of a plant system
WHAT IS XYLEM?
Xylem consists mainly of xylem vessels which is a hollw tube stretching from the root to leaf. The xylem vessel is made up pf many dead cells.While Xylem is known as the more identifiable cells,tracheids and vessel elements, tend to stain red with Safranin-O. Tracheids are the more primitive of the two cell types, occurring in the earliest vascular plants. Tracheids are long and tapered, with angled end-plates that connect cell to cell. Vessel elements are shorter, much wider, and lack end plates. They occur only in angiosperms, the most recently evolved large group of plants.
FUNCTION OF XYLEM
1st: Xylem conducts water and dissolved mineral salts from the roots to the stems and leaves.
2nd:Xylem provide mechanical support for the plant.
The inner walls of xylem vessels are strengthened by deposits of a substance called
which may be deposited in the form of rings or spirals.
plant diagram ( focus on the lignin)
How the structure of xylem related to its functions?
-The xylem vessel has an empty lumen without protoplasm or cross-walls which reduces water resistance flowing through it.
-Its walls are thickened with lignin which is a hard and rigid substance that prevents collapse of the vessel.
What is Phloem?
Phloem consists mainly of sieve tubes and companion cells. A sieve tube consists of columns of elongated, thin-walled living cells called sieve tube cells/elements. The cross-walls separating the cells have lots of ninute pores and look like a sieve thus re called sieve plates.
A mature sieve tube cell has only a thin layer of cytoplasm inside the cell.This cytoplasm is connected to cells above and below through sieve plates.
Each sieve tube cell has lost its central vacuole, nucleus and most organelles thus are said to have degenerate protoplasm.Thus, it has a companion cell beside it, carrying out the metabolic processes to keep it alive.
Companion cell is a narrow, thin-walled cell with many mitochondria, cytoplasm and a nucleus.
Companion cells provide nutrients and help the sieve tube cells to transport manufactured food.
plant phloem with sieve tube cell
Function of Phloem.
Phloem conducts manufactured food ( sucrose and amino acids ) from the green parts of the plant, especially the leaves to other parts of the plant.
How the structure of phloem related to its functions?
-Companion cells have many mitonchondria which provide the energy needed for the companion cells to load sugars from the mesophyll cells into the sieve tube cells by active transport.
-The holes in the sieve plates allow rapid flow of manufactured food substances through the sieve tubes.
Phloem and xylem are complex tissues in plant that perform transportation of food, water, minerals, and organic materials. In plants they are the vascular tissues and together form vascular bundles
Xylem is predominantly formed by tracheary elements like tracheids and vessels. Phloem consists of sieve tubes, companion cells, bast fibers
Primary xylem originates from the procambium and secondary xylem originates in vascular cambium during primary growth and secondary growth respectively. Phloem originates from meristematic cells in vascular cambium- primary phloem originates from apical meristem and secondary phloem originates from vascular cambium
Sap of xylem contains water, inorganic ions, and few organic chemicals and sap of phloem contains water and sugars
In xylem water travels by bulk flow and not through the process of cell diffusions. In phloem due to diffusion gradient water flows into cells and phloem sap moves from source of organic substance to sugar sinks by turgor pressure. Movement of water and minerals in xylem is facilitated by negative pressure while in phloem transportation is due to positive hydrostatic pressures.
Xylem consists of tubular structures with hard walled cells and phloem consists of tubular structures with soft walled cells
Xylem forms vascular bundles with phloem and provides mechanical strength to plant due to presence of lignin in cell walls.
Elements of phloem are Sieve tubes, phloem parenchyma, companion cells, bast fibers, and intermediary cells and elements of xylem are Trachieds, xylem vessels, xylem parenchyma, and xylem sclerenchyma.
Tissues of phloem are living tissue but xylem cells are dead at maturity
Movement of substances is bidirectional in phloem and it is Unidirectional (only upward) in xylem.
Phloem is involved in transport of food and nutrients from the leaves (source) to storage organs and growing parts of plant. Xylem is involved in transport of Water and mineral from roots (source) to aerial parts of the plant.
table summarising the differences of xylem and phloem
diagram showing the differences between xylem and phloem
1.The xylem and phloem are grouped together to form
2. The vascular bundles are arranged in a ring around a central region called the
3. The phloem lies outside the xylem with a tissue called the cambium between them. Cambium cells can divide and differentiate to form new xylem and phloem tissues, giving the rise to a thickening of the stem.
4. The region between the vascular bundles and the epidermis is the cortex and the pith serve to store up food substances, such as starch.
5. The stem is covered by a layer of cells called the epidermis. The epidermal cells are protected by a waxy, waterproof cuticle that greatly reduces evaporation of water from the stem.
into the vascular tissue in root
root cut longitudinally
In a dicotyledonous root, the xylem and phloem are not bundled together. Instead, they alternate with each other.
The cortex of the root is also a storage tissue.
The epidermis of the root is the outermost layer of cells. It bears root hairs. It is also called the piliferous layer.
Each root hair is tubular outgrowth of an epidermal cell. This outgrowth increases the surface area to volume ratio of the root hair cell. The absorption if water and mineral salts is increased through this adaptation
-The xylem and pholem are grouped together in a leaf to form the vascular bundle which will include supporting and protective tissues.
Each root hair is a fine tubular outgrowth of an epidermal cell. It grows between the soil particles, coming into close contact with the water surrounding them.
The film of liquid surrounding each soil particle is a dilute solution of mineral salts
The sap in the root hair cell is a relatively concentrated solution of sugars and various salts. Thus, the sap has a lower water potential than the soil solution. These two solutions are separated by the partially permeable cell surface membrane of the root hair cell. Water enters the root hair by osmosis.
The entry of water dilutes the sap. The sap of the root hair cell now has a higher water potential than that of the next cell. Hence, water passes by osmosis from root hair cell in to the inner cell
Similarly, water passes from one cell to the next cell of the cortex. This process continues until the water enters the xylem vessels and moves up the plant.
path of water in the root hair cell
-The root hair is long and narrow. This increases the surface area to volume ratio which in turn increases the rate of absorption of water and mineral salts by the root hair cell.
-The cell surface membrane prevents the cell sap from leaking out. The cell sap contains sugars, amino acids and salts. It has a lower water potential than the soil solution. This results in water entering the root hair by osmosis.
root hair cell
The living cells around the xylem vessels in the root use active transport to pump ions into the vessels. This lowers the water potential in the xylem vessels. Water therefore passes from the living cells into the xylem vessels by osmosis and flows upwards. This is called root pressure
Root pressure alone is not sufficient to bring water up to the leaves in tall trees
.Other factors at work include capillary action and transpiration pull
an experiment to demonstrate root pressure
-Water tends to move up inside very narrow tubes due to the interactions between water molecules and the surfaces of the tubes.This effect is called capillary action. Since the xylem vessels in the plant are very narrow tubes, capillary action helps in moving water up to the vessels.
-The force of attraction between water molecules is called the cohension. Water tends to stick or adhere to the inner surface of the xylem vessel just above the water surface attracts water molecules upwards by adhesion. The water molecules by the force of cohesion. This causes water to be drawn up to the vessels. This effect is called capillary action.
-Even though capillary action plays a part in the upward movement of water in small plants, it cannot acount for water rising up a tall tree.
experiment to show capillary action
-Green plants are continuously absorbing water from the soil. Not all the waterabsorbed will be used by the plant. The excess water, together with the water produced by the cells during tissue respiration, has to be removed. Excess water is mainly removed when it evaporates from those parts of the plant thatare above the ground(aerial).
-Transpiration is the loss of water vapour from the aerial parts of a plant, especially through the stomata of the leaves.
-Transpiration pull is the suction force caused by transpiration. It is the main factor that causes the movement of water up the xylem. Transpiration pull is the strongest force that causes water to rise up the leaves of tall trees.
experiment to show transpiration in plant
experimental set up to show transpiration in plant
-Transpiration pull draws water and mineral salts form the roots to the stems and leaves.
-Evaporation of water from the cells in the leaves removes latent heat of vaporisation. This cools the plant, preventing ot from being scorched by the hot sun.
-Water transported to the leaves can be used in photosynthesis; to keep cells turgid; and to replace water lost by the cells. Turgid cells keep the leaves spread out widely to trap sunlight for photosynthesis
importance of transpiration
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