TRANSPORT OF MATERIALS IN A FLOWERING PLANT

The parts of a flowering plant primarily concerned with transport of materials are the root hairs, the cortex of the root-hair region, wood vessels and sieve tubes of the phloem. We might also mention the transport of oxygen, carbon dioxide and water vapour across the stem of woody plants by way of the medullary rays connecting the living tissues with the outside air.

Movement of Water

It is most convenient when dealing with the movement of water, etc., from the roots to the leaves to consider first what happens to it in the leaves. Water in those cells in the spongy tissue which have air spaces around them evaporates into the air spaces and the vapour diffuses out of the leaves through the stomata into the outside air, the process being called transpiration. Evaporation leaves a more concentrated solution in the cells of the spongy layer causing water to pass into these cells from neighbouring ones by osmosis. These in their turn obtain water from wood vessels of the veins, where it is in unbroken columns, so that whenever water is absorbed into the leaves more must travel along these columns to replace it. As water coheres extremely well, the contents of each column rise as a whole. Evaporation, then, prior to transpiration, sets up a sucking action on the water in the wTood vessels, which is termed transpiration pull. The narrowness of the wood vessels results in considerable resistance to the movement, but causes capillary action to occur.

Each root hair is a portion of an epidermal cell of a young root, which grows out between the particles of soil. Its protoplasm is bounded by a cell wall, which is moistened outside by soil water. The fluid inside the root hair is a more concentrated solution than the soil water outside and therefore osmosis must occur, the water passing into the root hair. The presence of osmotic substances, e.g. sugar, in the cells of the cortex of the root causes this water to pass across the cortex to the central cylinder where, since the cell walls here are of cellulose and are therefore permeable to water, it can readily pass into the wood vessels. The water now rises up the vessels as the result of a force called root pressure. This gives rise to the phenomenon of bleeding when the stem of a plant is cut just above ground level. This force is not always present, however, and in any case is inadequate to raise water more than a short distance up the stem. A more powerful force in raising water is Transpiration Suction.

Experiment 52—To show how Water passes from Cell to Cell in a Plant by Osmosis

A medium-sized potato is peeled. One end is cut off squarely so that the potato can be stood vertically in a basin containing I in. depth of water. A hole 1 in. deep and about 1 in. in dia-

Suear solutionmeter is scooped out of the upper end.

After standing the potato in the water, the hole is one-quarter filled with a strong solution of cane sugar. After some hours the hole will be found to be full of liquid and overflowing. The sugar solution has removed water from the cells nearest the hole, and these in turn from cells deeper down, and so on until those nearest the water absorb water from the basin. In this way water passes from the basin into the hole by osmosis from cell to cell, the removal of water from each cell increasing its power to absorb more water.

This experiment helps us to under- stand how water passes from the root hairs across the cells of the cortex of the root and into the vessels, though not providing a complete explanation of a process not yet fully understood ; how the cells of the mesophyll of a leaf obtain more water from that in the vessels as they lose water, not by osmosis into a stronger solution, but by evaporation.

Experiment 53—To shoiu Transpiration Pull

The apparatus is fitted up, certain precautions being necessary, particularly to ensure that there is no air in the apparatus. The end of the shoot is cut under water to prevent air from entering the wood vessels.

To prevent the outer tissues from becoming damaged on passing it through the rubber bung, a cork borer of a slightly larger diameter than that of the shoot is passed through the hole in the bung from the under side until it just projects from the hole. The shoot is then inserted into the cork borer. The latter is then withdrawn, leaving the shoot firmly held in the bung. After inserting the bung into the tube the latter is filled with water, a finger is placed over the end and the tube is inverted under mercury in a strong glass vessel. The finger is then removed.

The whole apparatus is now placed in a draught. As water evaporates from the leaves, the mercury rises in the tube showing the upward pull exerted..

Experiment 54—To show the Routes traversed by Water from the Root

A small plant such as groundsel is dug up and all soil is washed from its roots in running water. Lumps of soil must not be pulled off but gently squeezed, in order to damage the delicate root structures as little as possible. It is placed with its finer roots in a beaker containing red ink diluted with an equal volume of distilled water. When red lines appear along the veins of the leaves, sections are cut of leaf stalk, stem and that portion of the root not placed in the ink.

These are examined with a magnifier. The path coloured red will be found to correspond with that of the wood vessels.

Young broad bean plants and narcissus also show excellent results.

Experiment 55—To show Root Pressure

The aerial portion of a geranium or fuchsia which has been growing in a pot for a long time is cut off to leave a stump 8 to 10 cm. long. The sides of the stump are well vaselined except near the cut. A rubber tube is fitted closely over this and it is bound on with twine. The tube is filled with water and a 199mercury pressure gauge fitted into it..

The pot is well watered and placed in a warm place. The readings of the limbs of the gauge are taken at regular intervals.

Experiment 56—To show Guttation

If transpiration is hindered by placing a low-growing potted plant, e.g. grass or nasturtium or seedlings of cereals, e.g. wheat, in a water-saturated atmosphere under a bell jar, drops of water are seen to exude from the edges of the leaves. This is called guttation, and is due to water forced out of the ends of the veins by root pressure.

The rate at which transpiration occurs depends upon the temperature, the humidity and the motion of the surrounding air, and also the condition of the stomata. Much heat energy is necessary to convert water into water vapour, so that on warm days the process occurs more rapidly than on cool ones. In fact, it may occur so rapidly in hot weather that the natural turgidity of cells in the leaves due to water pressure is so reduced that the leaves lose their firmness and wilt. A higher temperature also enables diffusion to occur more rapidly, and furthermore, the saturation value for the atmosphere rises.

When the air is saturated with water vapour no transpiration can occur, even should other conditions favour it. This condition, however, rarely obtains except with small plants growing close to the ground.

Transpiration will most readily occur when the concentration of water vapour in the air is low, I.e. when the air is dry, and is speeded up still more if there is a breeze. The air cannot easily become saturated around a plant unless the air around it is still.

The condition of the stomata of leaves is most rapid about noon in potato and 1 P.M. in maize. It has been shown, however, that the degree of opening or closing of the stomata does not greatly affect the rate of transpiration until the stomata are almost closed. The closing of the stomata at night largely contributes to the recovery of turgor by plants which have wilted through excessive transpiration during a hot summer day. Since transpiration practically ceases when the stomata close, water absorption by the roots during the night enables the water loss of the previous day to be made up..

Experiment 57—To show that Water Vapour is given off by the Aerial Parts of Plants

A waterproof bag made of rubber or of cellophane is tied round the pot and stem of a potted plant. This is placed inside a large bell jar which stands on a vaselined glass plate, and is left, preferably partly exposed, in a sunny place. Later, drops of liquid are to be seen on the inside of the bell jar away from the light. On testing, the drops of liquid will change anhydrous copper sulphate from white to blue, proving them thereby to be water.

Experiment 58—To show Transpiration by Means of Cobalt Chloride Papers

Some filter papers are soaked in a solution of pink cobalt chloride solution and dried. When required for use, a pair of these papers is taken and warmed gently until they turn bright blue.

A leaf is picked and placed between this pair of papers, and the whole sandwiched between two pieces of glass to protect the papers from the water vapour in the air. In a short time a pink patch will appear on the paper in contact with the under side of the leaf, corresponding in shape and size with those of the leaf. This shows that water vapour has passed out of the leaf by transpiration, turning the blue anhydrous cobalt chloride to the pink hydrated salt ; that transpiration occurs through the stomata and not through the cuticle of the leaf. Examination of a strip of epidermis taken from the upper and from the under side of the leaf will confirm this, since stomata will be seen to exist only in the lower epidermis.

Experiment 59—To measure the Rate of Transpiration of a Complete Plant

A plant, such as a green-house geranium, grown in a pot, is well watered and stood in an outer aluminium vessel. Sheet rubber is tied round the stem and round the aluminium vessel. It is weighed at regular intervals of, say, half an hour throughout the day. The only loss of weight can be by way of the stems and leaves of the plant, since the sub-aerial parts are in an airtight apparatus. The experiment works best if the pot is stood in sunlight by an open window.

Experiment 60—To measure the Rate of Transpiration of a cut Shoot

The shoot is cut under water and inserted into the rubber bung of the apparatus, using method in Expt. 53. It is left for some time to allow all the vessels to become full of water. The apparatus is flooded to sweep out air bubbles, by opening the tap of the funnel. For this reason the inner end of the capillary tube must not be below the bung. The tap is closed and air enters the end of the capillary tube as the water column is sucked along it. The time taken for the end of the column to pass from one graduation to another on the capillary tube is determined, the tube having previously been graduated, e.g. by placing a scale behind it.

This apparatus is excellent for determining the effect of conditions upon the rate of transpiration, e.g. in cool shade out of doors, in shade in the laboratory, in sunshine in a sheltered place and in sunshine in a draughty place, and in a warm humid atmosphere such as a green-house or a fume cupboard in which a beaker of water is gently boiling.

To use the apparatus for a small complete plant, the bung must be split across the hole into which the shoot fits. The shoot must be well vaselined where it passes through the bung.

Strictly speaking this experiment deals with rate of water intake rather than rate of transpiration.

Experiment 61—To compare the Water-intake ¦with the Water-loss of a Plant

A plant with a well-developed main stem is used. A split bung is now fitted round the stem and lightly inserted in the neck of the bottle. Vaseline is used to make all joints airtight. The burette is fitted and tap water poured in until all air has been replaced in the bottle. The bung is now firmly pushed in and made airtight. The apparatus is left for two days to allow the plant to ’settle down, ’ water being added if necessary. The burette is nearly filled, a few drops of olive oil poured in to prevent evaporation, the reading of the level noted and the whole apparatus weighed and put in a sunny place. The reading of the level of the burette and the weighing are repeated at corresponding times daily for four or five days.

Since 1 c.c. of water may be taken to weigh 1 gm., the rate of absorption may be compared with the rate of transpiration. Any difference is due to the fact that some of the water is retained by the plant, some of it being built up into other compounds to form a part of the plant.

The object of leaving soil on the roots is to keep them under conditions as natural as possible—if rather wet; using tap water is to supply the roots with the small quantity of oxygen necessary for their respiration. Since the experiment takes several days this is important.

TRANSPORT OF ELABORATED MATERIALS

Experiment 62—To show the Path of Transport of Food

A willow twig is ’ringed ’by removing a complete circle of bark about in. wide and extending inwards as far as the cambium, at a distance of a couple of inches from the lower end. The twig is then stood in water with the ’ringed ’portion completely immersed. After some time adventitious roots appear above the ring. This shows that food materials travel in the phloem. Food coming down from the leaves is unable to pass down below the ring for the formation of new roots.

While this is not an absolute proof that elaborated materials pass via the phloem only, since some of the cortex external to the phloem has been removed as well, it has been conclusively proved that the phloem is the sole channel for the transport of these materials. The actual channels are the sieve tubes. That these contain protein material can be shown by warming longitudinal sections of a marrow stem passing down the vascular bundles, with Millon ’s reagent. On examination under the microscope, a red precipitate can be seen in the sieve tubes showing the presence of protein.

In spring when the leaves of trees are unfolding from the buds, sugar and other foods stored in the medullary rays pass upwards in the rising sap in the vessels of the xylem. It is by tapping the maple trees to collect this sap in spring that the Canadians obtain the liquid which on evaporation yields maple syrup.

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