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Why is there a need of special transport system?
Over short distances, means of transport are rapid and efficient ie: osmosis, diffusion, endocytosis etc. But in multicellular organisms which have a large surface area to volume ratio; these means are sufficient as cells may be too widely separated from each other. Therefore for these practices to be adequate, specialised long distance transport systems are necessary.
Materials are generally moved by a mass flow system, being the bulk transport of materials from one point to another as a result of pressure difference between the two points.
Example of flow systems in plants and animals are;-
  • In plants- Vascular system.
  • In animals – Alimentary canal, respiratory system, etc.
A vascular system is one which has tubes which are full of fluid being transported from one place to another.
In animals the blood system is a vascular system.

In plants the xylem and phloem form vascular systems.
  1. Osmosis: This is a movement of water molecules from a region of higher water potential or lower solute potential to a region of lower water potential or higher solute potential through a deferentially permeable membrane.
  2. Diffusion: This is a movement of materials from a region of higher concentration to a region of lower concentration.
  3. Active transport: This is a transportation of materials against concentration gradient. Due to this, the process involves the consumption of energy. Any part of the body where active transport occurs is characterized by;
a) Presence of numerous mitochondria.
b) High rate of metabolism.
c) High concentration of ATP.
  • Since active transport involves the use of energy, the materials transported actively move faster than those transported passively.
Significance of transportation system
The system of transportation of materials is important for:-
  1. Distribution of food materials in the body.
  2. Carriage of excretory wastes from their sites of synthesis.
  3. Carriage of hormones from their respective glands to their target organs.
  4. Distribution of antibodies.
  5. Carriage of respiratory gases.
  • The movement of substances through the conducting or vascular tissues of plants is called Translocation.
Important application of the study of Translocation:
  • It is useful to know how herbicides, fungicides, growth regulators and nutrients enter plants and the routes that they take through plants, in order to know how best to apply them and to judge possible effects that they might have.
  • Plant pathogens are sometimes translocated, and such knowledge could influence treatment or preventive measures.
Terms used:
(i) Water potential, symbol Ψ, Greek latter psi.
  • The term is used to describe water movement through membranes. It can be described as the tendency of water molecules to move from one place to another. The higher (less negative) the water potential, the greater tendency to leave a system.
Factors affecting water potential of plant cells are:-
  1. Solute concentration and
  2. Pressure generated when water enters and inflates plant cells.
They are expressed in terms of Solute and Pressure Potentials respectively.
  • Pure water has maximum water potential (zero).
  • Water always moves from a region of higher Ψ to a region of lower Ψ.
  • All solutions have lower Ψ than pure water, therefore negative values of Ψ
(ii) Solute potential, Ψs
  • The effect of dissolving solute molecules in pure water is to reduce the concentration of water molecules and hence to lower the water potential.
  • Solute potential is a measure of the change in water potential of a system due to the presence of solute molecules.
(iii) Pressure potential, Ψp
If pressure is applied to pure water or a solution, its water potential increases. This is because the pressure tends to force water from one place to another.
Ψ = Ψs + Ψp
(iv) Plasmolysis and Turgidity
If a plant cell is in contact with a solution of lower water potential than its own contents, then water leaves the cell by osmosis through the cell surface membrane. Consequently, the protoplast shrinks and eventually pulls away from the cell wall. The process is called plasmolysis and the cell is said to be plasmolysed.
The point at which plasmolysis is just to happen is called Incipient plasmolysis. At this point, the protoplast has just ceased to exert any pressure against the cell wall, so the cell is Flaccid. Water will continue to leave the protoplast until its contents have the same Ψ as the external solution. No further shrinkage then occurs.
If a plasmolysed cell is placed in pure water or a solution of lower solute potential or higher water potential than the contents of the cell, water enters the cell by osmosis. As the volume of protoplast increases, it begins to exert pressure against the cell wall and stretches it.
The pressure inside the cell rises rapidly, the pressure is called the Ψp. As the Ψp of the cell increases due to water entering by osmosis, the cell becomes turgid.
Animal cells have no cell wall and the cell surface membrane is too delicate to prevent the cell expanding and bursting in a solution of higher Ψ. They are therefore protected by Osmoregulation.
  1. What occupies a space between the cell wall and the shrunken protoplasts in plasmolysed cells?
  2. What is the Ψp of a flaccid cell?
  1. The external solution, since the cell wall is freely permeable to solutions.
  2. Zero. The protoplast is not exerting pressure against the cell wall.
  • In higher plants, the materials are transported by the vascular tissues. Which are of two types:-
  1. The xylem and
  2. The phloem.
(i) The xylem tissue
This is a plant vascular tissue which is mainly concerned with transportation of water and dissolved mineral salts through the plant.
Structure of the xylem
The histology of the xylem tissue reveals the presence of four types of cells.
Note: The only conducting cells are the vessels and tracheids.
Structural features:
-They are more or less elongated cells with tapering ends.
-They have secondary thickened or lignified walls with a variety of pits (simple or bordered).
-They are not perforated.
-They are dead at maturity ie: they lose all the protoplasmic contents leaving an empty lumen.
Tracheids are present in all vascular plants, but in the coniferophytes they are only xylem conducting cells
– These are perforated elements that aggregate into files of cells connected to one another by means of perforations.
-The vessel members are more specialised than the tracheids and they are characterized by the following features:-
    • They have secondary thickened wall.
    • They are dead at maturity.
    • They are shorter and wider.
    • They have bordered pits along their sides.
    • They have perforated plates.
Role of vessels and tracheids
  • The vessels and tracheids conduct water and dissolved mineral salts through the plant ie: from the roots to the shoots.
Adaptations of the Xylem (vessels and tracheids) to transport
  1. Both have long cells joined end to end. This allows the flow of water and dissolved mineral salts in a continuous column.
  2. The end walls of the xylem vessels have been broken down forming uninterrupted flow of water from the roots to the leaves. Even in the tracheids where the end walls are present, larger bordered pits reduce the resistance of flow due to the presence of end walls. Absence of end walls in the vessels and presence of bordered pits in tracheids facilitate easy flow of water as resistance to flow is reduced.
  3. There are pits at particular parts in the lignified walls. These allow lateral movement of water and mineral salts where this is necessary.
  4. Narrowness of the lumen of vessels and tracheids increases the capillarity force.
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    The walls are lignified (for strength) making them especially rigid to prevent them from collapsing due to large tension force set up by the transpiration pull.
  6. Impregnation of the walls with lignin material increases the adhesion of water molecules which helps the water to rise up the plant by capillarity.
  7. Loss of protoplast in the vessels and tracheids leaves an empty lumen which forms a continuous tube as one cell rests on top of the other.
  8. Since the conducting cells of the xylem tissues are dead, their materials are transported through them passively and this minimizes energy consumption.
Question: Describe the histology of the xylem conducting cells and show how they structurally relate to their function.
-These are elongated, slender, thick walled and non conducting cells.
-They are thought to have been evolved together with the tracheids and they function as supporting elements.
-They also facilitate lateral movement of materials and they sometimes store food.
-These consist of simple undifferentiated living cells.
-They have lignified pitted walls and they are frequently arranged in radial sheets to form rays.
-They function as pathways for lateral movement of materials and they sometime store food.
(ii) The phloem tissue
  • It is chiefly concerned with translocation of photosynthetically manufactured food from the autotropic parts (sources) to the heterotropic parts (sinks) of the plant.
Structure of phloem
The histology of the phloem tissue reveals the presence of the following structural cells:-
  1. Sieve element.
  2. Companion cells.
  3. Phloem parenchyma.
  4. Phloem fibres and sclereids
-These include the sieve cells and sieve tube members.
  • These are associated with the sieve tube members only in the Angiospermophytes. They are highly specialized parenchyma cells.
  • They arise from the same meristematic initial with the sieve tube cells.
  • They contain nucleated dense cytoplasm in communication with the cytoplasm of sieve tube member by means of plasmodesmata in the pitted areas of the thin dividing walls.
  • These contain stored carbohydrates and accumulation of tannins and resins.
  • The phloem parenchyma is always in communication with the sieve elements and companion cells by the adjacent sieve areas.
  • These occur in both primary and secondary phloems.
  • The walls may be lignified or not but generally pitted with simple or bordered pits thus facilitating lateral movements of food substances.
1. The sieve tube elements when mature do not have the nucleus, no ribosomes, no golgi b
odies, no tonoplast. There are no mitochondria and there is a very little peripheral cytoplasm. However, the cells remain living since they

are connected to the companion cells
2. The companion cells have a dense cytoplasm with a nucleus, mitochondria and ribosome. The cells are very metabolically active.
3. The sieve tube members and the companion cells are in communication with one another by means of a large plasmodesmata.
Function of the phloem tissue:
  • The role of the phloem (sieve tube) is to carry food substances from the leaves to the other parts of the plant.
Adaptations of the phloem:
  1. The sieve elements are tubular to allow the passage of food substances.
  2. There are sieve plates with various pits to facilitate the passage of food from one cell to another.
  3. The pits in the sides together with plasmodesmata facilitate lateral movements of food.
  4. The mitochondria in the companion cells provide energy necessary for active transport of food.
  5. The tubes in the sieve elements are very narrow. This increases the pressure which in turn facilitates rapid transportation of food.
  1. Describe the structure of the phloem tissue and show how its structure relates to its function.
  2. Describe the histology of the plant vascular tissue.
NECTA 2001 P1 12
(b) How is the structure of xylem tissue suited to its function of transporting water.
Movement of materials across the root
  • Water and dissolved mineral salts from the soil, enter the root by osmosis across the epidermis of the root hair cells.
  • In the root, these materials move through three different routes/pathway namely:-
  1. Apoplastic pathway.
  2. Symplastic pathway.
  3. Vacular pathway.
I. Apoplastic pathway
  • The apoplastic pathway is found throughout the plant. However, in the root endodermis, it is prevented by the water proof substances called Suberin or Casparian strips.
  • Due to the presence of casparian strips, the moving water and dissolved mineral salts are forced into the living protoplast of the endodermis as the only available route to the xylem. This in turn, causes active secretion of salts into the vascular tissues from the endodermal cells. This makes water potential in the xylem lower (more negative) thereby increasing more chances for water to move into the xylem.
Significance of the casparian strips:
  1. They increase the chances of water moving into the xylem. This is because as they force water into the living protoplast of the endodermal cells, they cause salts to be actively secreted into the vascular tissue (xylem)from the endodermal cells. This makes water potential in the xylem lower (more negative), the result of which water moves into the xylem following a water potential gradient.
  2. They prevent an apoplast movement of water and dissolved mineral salts and therefore water and salts must pass through the cell membrane under the cytoplasmic control of the endodermal cells i.e They ensure the control of moving water by the living cells.
  3. They regulate salts movement and may be protective measures against entry of toxic substances example Fungal
  4. They serve the life of the cell when it plasmolyses as the cell surface membrane remains held in position at the strips although other parts detach.
  5. Since they are water proof bands, they normally regulate the amount of water to be admitted into the root.
  6. They are connected for maintenance of root pressure since they cause active secretion of ions/salts into the xylem vessel.
  7. It acts as an air tight dam in that it prevents water from being clogged with air.
  1. Symplastic pathway
  1. Symplast:-Is a system of interconnected protoplasts in the plant in which the cytoplasm are connected by the plasmodesmata, the cytoplasmic strands that extend from one cell to another through the pores in the cell wall.
  • The system in which the plasmodesmata link to ensure a living connection between the two neighbouring cells is called a Symplasm.
(ii) Symplastic pathway:- It is a pathway in which water moves from the cytoplasm of one cell to that of another through the pits by the aid of the plasmodesmata.
  1. Vacuolar pathway
In the vascular pathway, water moves from vacuole to vacuole. In this process, water moves across the cell surface membrane and tonoplast by the process of osmosis.
Diagram: Routes for water transport across cells


  1. Give an illustrated description of the pathway through which water and mineral salts pass from the root hair to the xylem vessel.
  1. Describe the significance of the casparian strips in the root endodermal cells.
Uptake of mineral salts and their transport across the root and through the plant
  • The plants take in the necessary mineral salts from the soil and their absorption is greatest in the region of root hairs. These are taken in either solution form or ions.
To explain the absorption of mineral salts, the following facts should be adhered to;
  • The cell membranes including the cell surface membrane and the tonoplast are not true semi permeable, but rather are differentially (selectively) permeable allowing some minerals to pass.
  • Minerals may be absorbed either passively or actively.
I.Passive absorption
If the concentration of a mineral in the soil solution is greater than its concentration in a root hair cell, the mineral may enter the root hair by diffusion.
II.Active absorption
If the concentration of a mineral in a soil solution is less than that in a root hair cell, it may be absorbed by active transport. Most minerals are absorbed in this way. The process is selective because active absorption requires energy the rate of absorption is dependent upon respiration.
  • There is a continuous system of interconnected cell walls, the apoplast in which water and any solute it contains enters by Mass flow and to a lesser extent by diffusion.
  • Although leaves can also absorb them if sprayed with a suitable solution, such sprays are called foliar feeds.
  • Water moves through the apoplast as a part transpiration stream. Transpiration stream is a movement (flow) of water from a root to the stomata.
Concisely but precisely, describe the process of uptake of mineral salts by the root.
  • Generally the concentration of mineral salts in the root hair cells is greater than that in the surrounding environment. This implies that the mineral salt enter the plants against the concentration gradient.
  • However, some ions are more concentrated than others. This suggests that ions are selectively absorbed by active transport involving the expenditure of energy from ATP.
  • Basically, ions (minerals salts) are absorbed from the soil by the root hair cells, where prime role is to increase the surface area for absorption. They are taken in together with water in order to form solutions after dissolving.
  • After absorption, they move to the xylem tissue through the apoplast pathway.
  • In the endodermis, there are water proof bands called casparian strips which concentrate the ions before passing to the xylem vessels.
  • In the xylem tissue, the ions together with water are carried upwards to the upper parts of the plant.
Upward movement of water and mineral salts
Once the mineral salts have been taken up from the soil, they enter the xylem together with water for their further transportation through the entire plant. This upward movement of water and mineral salts take place in the xylem vessels.
Forces governing upward movement of water and mineral salts
The forces that govern the upward movement of water and mineral salts through the plant include the following:-
  1. Capillarity.
  2. Root pressure.
  3. Transpiration pull.
  1. Capillarity
  • Capillarity (capillary forces) is the upward force that draws water up the plant against gravitational pull.
  • By this force, water moves up the plant through the narrow tubes of the xylem under the influence of pressure. The latter is due to narrowness of the xylem vessel tubes.
  • Capillarity is an important force as it causes water to rise high up in the tall trees.
The cohesion – Tension theory:
  • The cohesion theory describes the upward movement of water by capillarity through the plant.
  • According to this theory, the rising of water from the root is caused by evaporation of water from the lenticels.
  • Evaporation results in reduced water potential in the cells next to the xylem. Water therefore enters the cell from the xylem sap where it has a higher potential.
  • The xylem vessels are full of water and as water leaves the xylem, a tension is set up in the column of water.
  • This is transmitted back down the roots by cohesion of water molecule.
  • The latter have high cohesion due to their polarity and therefore tend to stick together being held by the hydrogen bonds.
  • Water molecules also tend to stick to the vessel walls by a force called adhesion. Thus, the tension in the xylem vessels builds up a force capable of pulling the whole column of water upwards by mass flow.
  • Water thus, rises in the fine capillary tubes due to high surface tension.
2. Root pressure
  • Water initially enters the root cells by osmosis from the soil solution and in that way it lowers the solute potential of root epidermal cells.
  • An active secretion of salts and other solutes into the xylem sap, tends to lower its water potential and for this reason water moves from one root cell by osmosis and then into the xylem.
  • The overall result is the creation of the root pressure which generates a hydrostatic pressure which causes a continuous upward movement of water.
  • However, this force alone is not sufficient to draw up water except in the slowly transpiring herbaceous plants where it causes guttation.
3. Transpiration pull
During transpiration; water is lost from the epidermal cells, as a result the water potential is lowered in the respective cells. Consequently, water is drawn from the xylem vessels whose sap has a higher water potential into the epidermal cells of the leaf.
  • This creates a continuous stream of water flow called a TRANSPIRATION STREAM which is a main route of upward movement of water and dissolved mineral salts. The force that pulls water upwards in favour of transpiration is called TRANSPIRATION PULL.
Transpiration is a process whereby a plant loses water from the epidermal cells of the leaves in the vapour form.
  • There are three types of transpiration:-
(a) Stomatal transpiration
  • This is a major way by which water evaporates from the plant leaves. It is a type of transpiration where by the plant loses water through the stomatal pores.
(b)Cuticular transpiration
  • This involves loss of water through the cuticle. In this way a very little amount of water is lost from the plant because the cuticle among other functions restricts water loss from the plant.
(c)Lenticular transpiration
  • This involves loss of water by the lenticels.
  • The latter are small slits in the stems and bark of trees for gas exchange.
Significance of transpiration in plants:
Transpiration is considered to be “Necessary evil”. This is because it is an inevitable but potentially harzadous process. It thus, has beneficial and harzadous effects.
  1. Beneficial effects
Transpiration is necessary in that;
  1. It is a means of transportation of water and dissolved mineral salts through a plant.
  2. It is a means of cooling the plant ie: evaporation of water from the surface of the plant eg: leaves has a cooling effect.
  3. It is a means of removal of excess water as waste product.
  4. Aids uptake of water and mineral ions.
  1. Hazardous effects
  • Transpiration is an “evil” process because excessive transpiration (loss of water) leads to dehydration of cells.
  • It also interferes with the process of photosynthesis, excretion, respiration etc. all of which require water.
  • As a consequence of excessive water loss the plant wilts and finally dies.
  • Structurally, the stomata pore is bordered by two sausage shaped guard cells. The latter have their inner walls being thick and less elastic whereas, the outer wall is thin and elastic (extensible).
  • The guard cells have chloroplast capable of photosynthesis. Around the guard cells are the epidermal and subsidiary cells.
Mechanism of stomata closure and opening:
The closure and opening of the stomatal pore is caused by change in turgor pressure of the guard cells. If water is drawn into catalyses the hydrolysis of ATP into ADP and Pi and energy is released.
  • The released energy used to pump K+ ions into the guard cell and H+ ions out of the guard cells. This also causes the inside of the guard cell to be alkaline.
  • The accumulation of K+ ion and glucose into the guard cells results into increased osmotic pressure in there.
  • The result of this increased osmotic pressure is the osmotic movement of water in to the guard cells from the epidermal cells.
  • Turgidity of the guard cells result into opening of the stomata aperture.
  • On contrary during the night, K+ ions are pumped out of the guard cells and H+ ions are pumped in. There is also an accumulation of CO2 in the intercellular spaces.
  • This result into increased acidity of the guard cells ie: Fall in the pH value. This fall in pH value favours the association of glucose forming starch in the guard cells while in the surrounding epidermal cell K+ ions (allophone) causes the accumulation of glucose.
  • The net effect is the osmotic movement of water from the guard cells to the epidermal cells. Thus loss of water from the guard cells causes them flaccid and hence closure of the stomatal pore.
  • Stoma is closed in the dark, but in the presence of light ATPase is stimulated to convert ATP to ADP and so provide the energy to pump out H+ from the guard cells. These protons return on a carrier, which also bring Cl with it. At the same time K+ enter guard cells.
  • As a result of this influx of ions, the water potential of the guard cells becomes more negative (lower) causing H2O to pass in by osmosis. The resultant increase in pressure potential causes the stoma to open.
  • In the dark, the movement of ions and H2O is reserved.
Describe the mechanism of stomatal closures and stomatal opening based on the osmotic pressure (Pressure flow) hypothesis.
  • The guard cells, the latter become turgid and stomatal pore opens. And when the cells are flaccid, the stomatal pore closes. The guard cells, the latter become turgid and stomatal pore opens. And when the cells are flaccid, the stomatal pore closes.
  • The guard cells have thicker inner inelastic walls and thinner elastic outer walls. During expansion they do not expand uniformly in all directions.
  • The thick and less elastic inner walls are less pulled out wards leaving an open between them.
How is the mechanism explained?
1. A traditional hypothesis the starch-sugar hypothesis suggested that an increase in sugar concentration in guard cells during the day led to their solute potential becoming more negative, resulting in entry of water by osmosis.
However, sugar has never been shown to build up in guard cells to the extent necessary to cause the observed changes in solute potential.
K+ ion and osmotic pressure theories:-
It has now been shown that potassium ions and associated negative ions accumulate in the guard cell during the day in response to light and are sufficient to account for the observed changes.
In darkness, potassium (K+) ions move out of the guard cell into surrounding epidermal cells. The water potential of the guard cells increases as a result and water moves out of the cells. The loss of pressure makes the guard cells change shape and stoma closes.
What causes K⁺ to enter the guard cells in the light?
Ans: K+ may enter in response to the switching on of an ATPase which is located into the cell surface membrane which pumps out H+ and K+ may then enter to balance the charge.
More explanations:-
  • During the day, the plant photosynthesizes by consuming CO2.
  • This reduces the concentration of CO2 in the intercellular spaces of the leaf.
  • This lowers the level of Carbonic acid and hence a rise in pH value ie: The cells become more alkaline.
  • This favours the conversion of starch into glucose which accumulates in the guard cells. At the same time the enzyme ATPase.
2. Using carbon – 14 isotope
  • If a plant with a ringed stem is supplied with CO2 containing C-14 isotope ie: 14CO2, the food substances accumulated above the ring appear to contain C-14. This suggested that the synthesized food is translocated through the phloem.
3. Using mouth parts of a feeding aphid
  • An aphid is an insect that uses its tubular needle – like mouth part to feed on the sugary solutions from the phloem sieve tubes.
  • If the feeding insect is anaesthesized with CO2 and the mouth parts are carefully cut so that the tube remains inserted into the phloem vessel, the food substances continue to move through the tubular needle of aphid.
  • Analysis of this solution reveals the presence of sugary substances and amino acids all of which are the products of photosynthesis.
  1. There are diunal variations in the concentrations of the glucose which are in turn reflected in the phloem sieve tubes.
Mechanism of Translocation by the phloem:
There is no one agreed mechanism by which food substances are translocated through the phloem. However there are various hypotheses that try to describe the mechanism of phloem translocation. They include:
A. Mass flow hypothesis (Münch 1930)
This is also called Münch’s hypothesis or pressure flow hypothesis. According to this hypothesis, food substances are translocated through the phloem in a mass flow mechanism.
Consider the munch model bellow:
  • Could the ions reach the xylem entirely by means of the apoplast pathway?
Ans: No, the endodermis is a barrier to the movement of water and solutes through the apoplast pathway. This is due to the presence of casparian strips which prevents further progress
  • To cross the endodermis, ions must pass by diffusion or active transport through the cell surface membranes of endodermal cells, entering their cytoplasm and possibly there vacuoles. Thus the plant monitors and controls which type of ions eventually reach the xylem.
Ions can also move through the symplast pathway. The final stage in the movement of mineral salts across the root is the release of ions into the xylem.
Once in the xylem, they move by mass flow throughout the plant in the transpiration stream.
The chief sinks, ie: Regions of use, for mineral elements are the growing regions of the plant, such as the apical and lateral meristerms, young leaves, developing fruits and flowers and storage organs.
Translocation of the manufactured food:
  • In higher vascular plants, food substances are translocated through the phloem.
Evidence to show that phloem translocates food:
(i) Ringing experiment.
A ring of tissue containing phloem was removed from the outer region of the stem, leaving the xylem intact. It was found that the lea
ves did not wilt, but growth below the ring was greatly reduced. This is because, movement of sugars down the plant were stopped without affecting passage of water upwards.
Description of the model
  • In the model, there is an initial tendency of water passing by osmosis into A and C. However, the tendency is greater for A than for C because the solution in A is more concentrated than that in C.
  • As water passes into A, a pressure potential (hydrostatic pressure) builds up in the closed system A-B-C forcing water out of C.
  • Mass flow of solution occurs through B along the hydrostatic pressure so generated.
  • There is also an osmotic gradient from A to C and eventually the system comes into equilibrium as water dilutes the contents of A and solutes accumulate at C.
Application of the model to the living plant
  • The leaves which make sugars during photosynthesis are represented by A. The synthesized sugars, lower the water potential of the leaf cells and consequently this fuses the flow of water into the leaves by osmosis through the xylem (D).
  • Due to hydrostatic pressure generated into the phloem (B), food from the source (A) to the sinks such as roots and storage organs (C) are transported in a mass flow system.
(1) Water is lost in form of vapour
-Water is lost in form of liquid.
(2) It occurs all the time
-It occurs only at night.
(3) It occurs in stomata, cuticle and lenticels
-It occurs in the hydathodes.
(4) It is favoured by high temperature, low humidity and light
-Favoured by high humidity, low temperature and darkness.
(5) Occurs in all plants
-Occurs only in members of the grass family.


– Transports blood away from the heart.
– Transport blood towards the hear
– Link arteries to vein. Site of exchange of materials.
– Tunica media thick and composed of elastic & smooth tissues.
– Tunica media relatively thin and only slightly muscular. Few elastic fibres.
– No tunica media.
– No elastic fibres.
– No semilunar valves (except where leaves heart).
– Semilunar valves present so as to prevent back flow of blood.
– No semilunar valves.
– Blood plow rapid.
– Blood flow slow.
– Blood flow slowing.
– Low blood volume.
– Much higher blood volume than capillaries or arteries.
– High blood volume.

– Blood oxygenated except in pulmonary artery.
– Blood deoxygenated except in pulmonary vein.
– Mixed oxygenated and deoxygenated blood.

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    Emason, March 9, 2024 @ 8:04 pm Reply


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