It is one row of adjacent, barrel-shaped parenchyma cells with no intercellular spaces.
The cells are covered by a cuticle. Some stomata may be found.
It is a narrow layer formed of several rows of cells.
The outer layer is formed of Collenchyma cells (parenchyma cells with corners thickened with cellulose) to support the stem but it may contain chloroplasts & thus named Chlorenchyma to take part in photosynthesis.
The inner layer is formed of parenchyma cells with intercellular spaces for aeration.
The last raw of the cortex is formed of thin walled parenchyma cells containing large quantity of starch known as starch-sheath.
The adaptation of the cortex to carry out its functions:
a- The presence of the collenchyma cells near the epidermis to support the stem & to prevent its bending by the action of winds
b-The presence of chloroplasts to make photosynthesis.
c-The presence of starch sheath to store food,
3- Vascular cylinder:
It exists near the epidermis to resist the action of winds.
It consists of the following tissues: -
• It is formed of a cylinder of parenchyma cells alternated with fibers.
• The fiber's walls are formed of lignified cellulose to support the vascular bundle.
b- vascular bundles
• They are arranged in a cylinder
• Each bundle triangular in shape with its base directed outwards.
• Each vascular bundle has the following tissues:-
It is directed outwards.
It consists of sieve tubes, companion cells and phloem parenchyma .
It carry the high - energy food from the leaves to all the plant parts.
It lies between phloem & xylem.
It is formed of meristematic cells with large nuclei.
Its function is to produce secondary phloem outwards and secondary xylem inwards, so that the thickness of the stem increases.
• It is directed inwards.
• Protoxylem is directed towards to the centre but Metaxylem
is directed outwards.
• Xylem vessels are separated by xylem parenchyma.
• Xylem consistes of vessels , tracheides & xylem parenchyma.
The xylem & the phloem are on the same radius.
1- Xylem vessels
• They are formed of vertical columns of elongated cylindrical cell
• They are joined end to end where their transverse walls disappear.
• The cells are dead & hollow as they are devoid of protoplasm.
• The walls of the vessels are thickened with lignin.
• Pits are unthickened areas at the lignified walls.
• The xylem vessels have different shapes according to different ways of thickening lignin deposit as strips.
• It has 2 main functions; The first is to support the vessels not to collapse.
The second is to leave the vessels pitted.
3- Xylum parenchyma
• Several rows of cells connect between xylem vessels in vascular bundle of stem.
• The root xylem is connected to the stem xylem and leaves xylem, The same in phloem.
The adaptation of xylem tissue to carry out its functions:
1- It is formed of elongated cylindrical cells joined end to end to form vessels
2- Their transverse walls disappear to facilitate the movement of H2O & salts.
3- The cells are hollow & devoid of protoplasm to carry H2O & salts.
4- Their walls are thickened with lignin to prevent the water to get out of the vessels and support the plant.
5- The unthickened Pits to allow the water to move at the two sides.
It is wide & occupies most of the stem volume.
It is formed of large parenchyma cells with intercellular spaces to store food.
Pith may disappear leaving the stem hollow as clover stems & bean plants.
5- Medullar rays:
They are extended between the vascular bundles joining the cortex with the pith.
The mechanism of transportation from the root to the leaves
H2O & salts are transported from the root to the leaves along xylem.
The xylem is formed of vessels & tracheids
The factors that are responsible for the ascent of sap
If the stem of a plant is cut above the soil, exudation of water appears at the cut surface. This phenomenon is called guttation or exudation.
This phenomenon occurs due to high osmotic pressure of the root hairs, absorption of water continues non-stop to push the water upwards along the xylem vessels. As water rises verticality exudation occurs. This is what we call root pressure
When the root pressure equals to the weight of the water column inside the
xylem vessel, water stops rising , Therefore the root pressure is limited
The root pressure can not explain the ascent of sap along the xylem vessels
Ans. Because: 1- Root pressure does not exceed two atmospheric pressure.
2- Some plants as Pinus (Conifers) have no any root pressure.
3- The force of the root pressure is affected by the environmental factors.
2- Imbibition theory:
The lignified cellulose of the xylem vessels has colloidal property, so they have the ability t imbibe water and the Imbibed water may diffuse upwards.
Imbibition can not explain the ascent of sap?
Ans. Because the experiments prove that the sap ascends through the cavity of the xylem vessels & not along their lignified walls.
3- The theory of capillary:
The capillary action is the movement of water in fine capillary tubes against gravity.
The diameter of the xylem vessels ranging between 0.02 to 0.5 mm.
The ascend of sap increase when the capillary of the vessels, increase.
Capillary can not explain the ascent of sap?
Water rises inside the narrowest vessels more than 150 cm. So capillary is considered as weak and secondary force affecting the ascent of sap
4- Cohesive force, adhesive force and transpiration pulls theory:
(By Dixon & Joly in 1895)
☺The theories explain the ascent of sap to the top of high trees (up to l00m).
☻Each force has a special role in the ascent of sap as folov;i
a- The cohesive force is due-to the strong attraction between the water molecules inside the vessels. This helps the water column to exist inside the vessel.
b- The adhesive force is due the strong attraction between water molecules & the walls of the vessels. This helps the water column to resist gravitational force.
c- the transpiration pull by which water rises inside the vessels. This helps the continues column of water to move upwards.
The conditions should be fulfilled in the xylem vessels in order to be capable for the transportation of the raw materials:
1- The vessels must be capillary tubes.
2- The walls should have high adhesive force to water molecules.
3- The water column should be devoid of air bubbles, the air bubbles cuts, the water molecules, so that the water column falls down due to gravity.
The failure of transporting a cultivated plant part after being moved from the soil & left exposed to the sun rays for long time?
Because water evaporates through stomata, then air enters through the cut surface to fill the xylem vessels breaking the water column. As a result the plant wilts & dies even after recultivation.
Explain how does the sap rise inside the xylem vessels?
Ans. This according to the following steps
Transpiration decreases the humidity in the air chambers opposite to the stomata
The mesophyll cells start to evaporate water to replace the lost water from the chambers So that the concentration inside the cells rises.
The cells now pull water from the neighbor cells, till the pulling water reaches the xylem vessels
Water inside the vessels are pulled up along vessels & tracheids of both stem & root as they are connected
When water is pulled up in the root vessels until reach the vascular cylinder of the root and also help in the lateral pull of water from the root hairs
Accordingly the concentration inside the root hairs rise thus water is absorbed from the soil.
Transport of manufactured food from the leaf to other parts of the plant:
The important of phloem:
Translocates the high - energy organic
substances produced by the leaf during
photosynthesis to all the plant part.
The structure of phloem:
It consists of sieve tubes and companion cells and phloem parenchyma.
1. Sieve tubes are elongated cells, arranged end to end, separated from each other by transverse walls called sieve plate which are perforated by tiny pores through which cytoplasmic strands extend from tube to another, they contain cytoplasm without a nucleus
2. companion cells Each sieve tube has a nucleated companion cell to regulate all vital activities of sieve tube as they have large number of ribosomes and Mitochondria.
Experiments to prove that manufactured food is transported along phloem
1) Rapidan & Bohr’s experiment: (1945)
Leaves of bean plant were exposed to CO2 formed of C14 (radioactive). The plant then was left to carry out photosynthesis.
Radioactive carbon was then detected inside the plant. It was detected inside the phloem vessels up & down the stem.
2) Mittler’s experiment:
Certain species of insects named Aphid feeds on manufactured plant food. The insect pears its proboscis through the epidermis of the plant until it reaches the phloem & starts to suck the food.
By separating the body of the insect from the proboscis, during its feeding,
& by analyzing the contents of the insect’s stomach it was found that this solution is formed of sucrose & amino acids.
Miller followed the proboscis to find out how deep it reached inside the plant
He found that the tip of the proboscis dip into the phloem sieve tubes which contains sucrose & amino acids..
Conclusion: The phloem vessels transport manufactured food up & down to all
the plant parts.
The mechanism of the transportation of the organic food compounds by the phloem:
1961 Thain & Canny could see cytoplasm strands extending along the sieve tubes.
They noticed that these strands are connected in different tubes the strands pass through the pores in the sieve plates, They also noticed that strands carry food particles along the tubes.
They concluded that transportation of manufactured food is due cytoplasm streaming inside the sieve tubes. By cytoplasm streaming the food moves from one tube to another up & down the phloem vessels passing through the pores in the sieve plates.
It was proved that cytoplasm streaming required energy (ATP). ATP is produced in a considerable amount in the companion cells & passes through the plasma membrane to enter the sieve tubes.
Cytoplasm streaming is affected by temperature as well as oxygen supply. Movement of the cytoplasm & thus transportation inside the phloem slows down in . the conditions of low temperature or lack of oxygen.