Transpiration in Plants and Animals, Water and Salts – The plants have two types of vascular tissues, namely the xylem and the phloem. The xylem tissue transport water and the dissolved minerals (inorganic salts) from the roots upwards to the stems and the leaves.
The end walls between the vessel element are broken down so that they form continuous, uninterrupted tubes, allowing water to be easily conducted upwards mainly due to transpiration pull, which is discussed later in this chapter.
‘‘Most of the gymnosperms and dicots lack vessels; instead they have tracheids. Each tracheid is an elongated, dead cell with ilgnified walls, and intact end walls.’’
Uptake of Water and Minerals by Roots
Water with dissolved salts enters the plants from the soil through the roots and is then transported to all parts of plant by xylem. The root hairs, arising just behind the root tip, are outgrowths of the roots. They enormously increase the surface area for the absorption of water and salts.
The mineral salts are dissolved in the soil water. Much of the water end minerals that enters the root is taken up through the root hairs (fig. 12.6). The sap water alongwith dissolved substances moving through the plant or present in the vacuole of the root contains a strong solution of solutes (sugars and various other inorganic and organic salts). Thus a concentration gradient is established between the root hairs and the soil water. So the water from the soil moves into the root hairs by osmosis. The entry of water into the root hairs dilutes its sap.
Now the sap of the root hair is moor dilute then that next cell. So the water from the root hair moves into root cells. The water from the epidermal cells then passes into the xylem vessels and ascends up the stem. This upward movement of water and salts is called ascent of sap. Some water Travels upwards along the cell walls and some through the cells themselves moving from one cell to another.
Practical work: to demonstrate the ascent if sap in plants
Take a medium-sized herbaceous potted plant. Uproot it and place it in a beaker containing red, eosin solution. The root must be completely submerged in the dye solution. Let the plant stay in the solution for about half an hour. Now cut transverse sections of the stem and leaves from the part of the plant that was out of the eosin solution. Examine the sections under the microscope. You will observe that xylem portions of vascular bundles would have become red by the eosin in the plant this indicates that water is conducted upward through the xylem vessels in the different organs.
Transpiration
The xylem vessels of tall trees transport abundant water from the roots to the leaves situated a height of several meters. What is the force that causes this movement?
Water is the major constituent of plant tissues. The plant absorbs large amounts of water from the soil. Of the water absorbed from the soil, only a small proportion of it is retained by the plants while the rest of it is passed off into the atmosphere. So, transpiration can be defined as” the loss of water by evaporation from the aerial parts (cells) of the plant, especially through the stomata of leaves.” As a consequence of loss of water by transpiration, plants require large amounts of water for their survival.
“Fig. 12.6 The-transpiration-stream”
Some of the water entering the leaves diffuses into the cells of the Mesophyll tissue of the leaves through their cellulose cell walls (as cellulose is freely permeable to water). The mesophyll cells are in contact with the air spaces in the leaf. The water from the cell of the mesophyll diffuses through their walls into the intercellular spaces from where it evaporates and diffuses into the atmosphere through the stomata.
The evaporation of water from the top of the leaf reduces the concentration of water but increases the concentration of solutes in the leaf cells. So because of higher osmotic pressure in the leaf cells form the xylem. The uptake of water from the top of the xylem lowers the hydrostatic pressure at upper region of the xylem vessels. Whereas at base of the vessels, this pressure at the base of the vessels produces a pull or tension which pulls the water from the area of high pressure (lower xylem) to the area of low pressure ( upper xylem).
The pull or of suction force thus produced in the xylem vessels by the evaporation of water (transpiration) from the leaf is called transpiration pull. As a consequence of the transpiration pull and the cohesion and adhesion of water molecules, the water moves up the xylem vessels as an unbroken column called transpiration stream. The xylem vessels are a continuous system of tubes from the roots, to the stem and the leaves.
Factors Affecting the Rate of Transpiration
The rate of transpiration depends on some environmental factors; such as temperature, wind, humidity, light, atmospheric pressure etc.
(i) Temperature and wind
The temperature-greatly affects the rate of transpiration. When the-temperature is high, rate of evaporation increases, the water molecules move rapidly than at low temperatures and also because warm air can hold more water vapours than the cold air. So, on a warm day the rate of-transpiration increases. Windy condition also increase the rate of-transpiration as wind removes water vapours from leaf and keep the concentration difference high between the leaf and surrounding air .
(ii) Humidity
In nearly dry condition or low humidity, the air around the plant will be dry; it will have low concentration of water than that inside the leaves. This difference in the concentration of water would result in an increase in the rate of transpiration. When there is more humidity in air, -transpiration would be considerably low; as the air is already saturated with water will diffuse out of the leaves.
(iii) Light
Light is an important factor in controlling the rate of-transpiration as it greatly influences opening and closing of stomata of leaves. During day-light, the stomata remain open and allow the water vapours from the leaves to diffuse out into the atmosphere. At night or during hours of darkness, the stomata usually close, and the rate of-transpiration is greatly reduced or it may even stop completely.
(iv) Atmospheric pressure
The atmospheric pressure also affects the rate of transpiration. Reduction in the atmospheric pressure enhances the rate of transpiration.
Importance of Transpiration
Transpiration plays an important role in the life of plants as it provides the forces or tension to pull water and minerals ions up the xylem vessels from the roots to the leaves. Sometimes, when the water supply is short, more water may be lost from the plants by transpiration than is absorbed from the soil.
In such a condition the cells loses water. As a result of this, the plant loses their turgor and become flaccid. The plant has the ability to withstand such a condition if it lasts for a short period. But if such a condition persists for long periods, the plant wilts even dies. In such conditions many plants have ability to control the rate of transpiration by closing their stomata.
Stomata
The stomata are very minute openings in the epidermis of the leaves. A stomata (singular of stomata) consists of a small central opening surrounded by a pair of modified epidermal, sausage- shaped, cells called guard cells. The guard cells regulate or control the size of the stomal opening, by changing their own shape. When the guard cells are turgid, their inner margins curve apart from each other because their inner walls are thicker than outer ones.
Thus by the curving if the inner margins of guard cells the aperture between them opens. On the other hand, when the guard cells loose their turgidity, they become flaccid and their inner margins become straightened and come to lie close together. In this way, the stoma is closed. If the stomata are closed, transpiration is almost completely stopped.
Measurement of the Rate of Transpiration
The rate of transpiration can be measured by many methods. It is generally believed that the rate of absorption of water from the soil is nearly equal to the rate of transpiration. Of the many methods for the measurement of the rate of transpiration, one makes use of potometer, a glass apparatus that works on above-mentioned principle.
Practical Work: Comparison of the rate of transpiration by potometer
Take a leaf shoot and fix it in of the arm of a thoroughly water- filled potometer. If there are any air bubbles, these should be excluded. All connection should be made air tight with the help of vaseline or plaster of Paris. Now introduce the air bubble in to the capillary tube of the apparatus. Place the setup at a well-lit place.
As soon as the transpiration starts, the air bubble will begin to move along the capillary tube, the rate of movement of the air bubble gives the measure of transpiration. Rate of transpiration can be compared under different condition such as light /shade; under the fan /away from fan etc.
Practical Work: Comparison of transpiration from too surfaces of leaf by cobalt paper
The lower surface of the leaf has more stomata than the upper surface, so more water transpires from the lower surface than the upper surface. This can be demonstrated by the cobalt chloride method. In this method the amount of transpiration of water is estimated by the change in colour of the cobalt chloride paper. When dried cobalt chloride paper is exposed to humid air or moisture it will gradually change its colour from blue to pink the normal the colour of dry (anhydrous) cobalt chloride paper is blue.
The cobalt chloride paper can be prepared by treating filter paper disc with 3% slightly acidic solution of cobalt chloride. After treatment the filter paper discs are thoroughly dried. In order to perform this experiment, take to equal sized discs of blue cobalt chloride filter papers place one disc on the upper and one on the lower surface of the leaf of a potted plant. You will see that after a few minutes, the cobalt chloride paper starts changing colour from blue to pink. You will note that the paper fixed on the upper surface of the leaf takes much longer time to turn pink from blue than that of the paper on the lower surface of the leaf. The rate of change of color indicates the rate of transpiration, which is much higher from the lower surface than from the upper surface. This is because of the presence of stomata on the lower surface.
Path of Transport of Organic Materials in Plants
Organic Materials in Plants: The green plants manufacture food in the leaves. From the leaves, the sugars need to be moved to the non-photosynthetic parts of the plants such as roots, stems and flowers for storage or consumption.
The manufactured food substances (sugars) are transported from the leaves to the other parts of the plant body by phloem tissue (mainly sieve tubes) as its structure is well suited for this function.
The transport of carbohydrates, mostly in the form of sucrose (sugar), from the leaves to all other parts of the plant is called translocation. In addition to sugars, the phloem also transports amino acids, growth hormones, and vitamins.
The mechanism of the translocation of these substances through the sieve tube cells of the Phloem is not yet clearly understood. Several hypotheses have been proposed to explain this mechanism. One of these hypotheses is the mass flow hypothesis (Fig. 12.9). many plant physiologists favour this hypothesis.
According to the mass flow hypothesis,
The sugars in solution can move freely through the Sieve tubes from the leaves to all non-photosynthetic parts of the plants.This movement is because of the difference in the concentration gradient at the place of manufacture of sugar i.e., the leaves and the other parts of the plant which need nourishment to carry out their activities.
As the food is manufactured in the leaves, the sieve-tubes in their veins are loaded with sugar. More water therefore enters then by osmosis. Consequently they have a high sugar pressure. Other parts of the plants have low concentration of sugar pressure. So the sugar will move from a region of high concentration (source) to the region of low concentration(sink).
The phloem stream, carrying the manufactured food, moves in the direction opposite to that of the transpiration stream. One objection against the mass flow hypothesis is that the phloem often transports substances in the opposite direction too i.e., up the stem to provide nourishment and energy to the leaves and shoots for their growth and also to the developing seeds and fruits.
But this objection to the mass flow hypothesis does not hold good if we recall the structures of the phloem tissue. The phloem tissue consists of many sieve tubes, not just one, and the fluids can move in opposite direction in two different but closely situated sieve-tubes. Here too, the movement of fluids in opposite direction would also depend on opposing pressure gradients in the two ends of sieve-tubes.
Transport in Animals
Transport in Animals – As you have already learned that in all leaving beings the nutrients and gases are transported to and from all parts of the body. This is essential to carry on various life processes. In case of unicellular and small multicellular organisms transport takes place by diffusion. However, in large multicellular organisms, as the distances between different body parts have increased, they need an elaborate and efficient system for transportation of materials. In large animals, such a system is called circulatory system in which a fluid circulates in all parts of the body. In many invertebrates this fluid is the haemolymph, where as in all vertebrates and in some higher invertebrates this fluid is the blood.
The circulatory system in animals is of two main types (A) open circulatory system and (B) closed circulatory system.
Open Circulatory system
Many invertebrates e.g. arthropods have open circulatory system [fig12.10 (a)].In this system the blood is pumped from the heart into the blood vessel. The blood vessels in turn, empty themselves into open spaces called sinuses. In the sinuses, the blood is in direct contact with the tissues, and after exchange of materials with the tissues it re-enters the heart for circulation again.
Closed Circulatory System
Closed circulatory system is more elaborate, complicated and efficient as compared to the open circulatory system. The closed circulatory system consists of a muscular, and contractile pumping organ, the heart with its incoming (veins) and outgoing (arteries) blood vessels. The blood remains confined in the blood vessels while circulating in the whole body e.g. earthworm, man etc.
In closed circulatory system, the heart pumps blood into the blood vessels (arteries) which take away the blood from the heart to the tissue. In the tissues, the arteries divide and subdivide into very fine branches, called the capillaries. The walls of capillaries are just one cells thick, and in them the blood is in close contact with the tissue cells. Exchange of materials with tissues is carried out here.
The capillaries join and form bigger blood vessels called venules. These venules in turn join to from the veins, which ultimately transport blood back to the heart.
General Plan of Circulatory System of Vertebrates
In vertebrates, the circulatory system is always of closed type. The closed circulatory system is further of two types, (a) single circuit circulation, (b) double circuit circulation.
For instance in fishes the circulation is of single type. In it, only the deoxygenated blood circulates through the heart. The deoxygenated or the venous blood from all tissues of the body enters the sinus venous, from where it passes into the single auricle or atrium. From the atrium it goes in the ventricle.
From the ventricle, the blood is pumped into the gills for oxygenation. The oxygenated blood from the gills is directly distributed to all parts of the body. As the blood circulates once through the heart, therefore, this type of circulation id called single circuit circulation.
In land vertebrates, with the introduction of lung respiration, double-circuit circulation evolved.
The evolution of double circuit circulation led to the division of atrium and ventricle, each into two chambers. The atrium divided into right and left atria (plural of atrium) and likewise the ventricle also divided into two chambers. In amphibians, the ventricle is not divided and in most reptiles, the division of the ventricle is incomplete. In some reptiles and in all birds and mammals, the division of the ventricle is complete. So in these animals, oxygenated and deoxygenated bloods are completely separated from each other and there is no mixing of these two types of blood. This increases the efficiency of the circulatory System in vertebrates is highly developed and among them mammals have the most efficient circulatory system.