RESPIRATION IN PLANTS

It is perhaps appropriate at this point to repeat once more the fact that gaseous exchange is not in itself the most important process in respiration, but precedes and sucoeeds tissue respiration, which is the essential feature in respiration.

Flowering Plants

The respiratory organs of flowering plants are root hairs and those parts above ground where ’stomata ’ and ’lenticels ’ are to be found. The structures of stomata and of lenticels have already been described. Each root hair grows out from one cell of the surface of the root, possessing an extremely thin surface membrane, through which many materials in solution readily pass. Such materials dissolve in rain, forming the soil water. It is well known that plants will not grow well in soils which have few air spaces. This is due to the fact that soil water does not contain sufficient oxygen for the requirements of the root hairs. Such a soil is one containing much clay. The dying of vegetation when soil becomes water-logged is due to the same fact. Oxygen in the soil water passes by liquid diffusion into the root hairs, passing along them into the living cell tissues of the root and up the root in the wood vessels to all other parts. Carbon dioxide diffuses out of the root hairs.

The presence of air inside the leaf is readily demonstrated.

Experiment 19—To show the Presence of Air Spaces inside Leaves

Various leaves are rubbed in running water so that air bubbles will not stick to them. Water is boiled in a beaker, a drop of hydrochloric acid being added to make the water clear if it is hard. The bunsen is removed, and when the bubbles of steam have ceased to rise from the water, the leaves are plunged into it, so that they are completely immersed. The following observations are made :—

Whether the bubbles appear immediately or not.

Whether more appear on the upper or under surface.

Whether there is any difference between the results obtained with shiny tough leaves and those which are dull and tender.

Whether any bubbles emerge from the stems of the leaves.

The materials in the individual cells pass from cell to cell by liquid diffusion, for the cellulose cell walls are neither water nor gas proof. From the cells of the spongy layer they can pass into the air spaces, if capable of existing in gaseous form at ordinary temperatures. Such are water, oxygen and carbon dioxide. Since the air spaces are connected with the exterior through the stomata, these materials can get out of the leaf by this route by gaseous diffusion. Similarly oxygen and carbon dioxide can diffuse in when the concentration of either of these gases is less inside the leaf than it is outside.

If young twigs of trees are treated as in the previous experiment the lenticels prove to be places where gaseous exchange can occur. They consist of masses of thin-walled cells through which gases rapidly diffuse and which are not covered in by the somewhat gas-proof layer, the epidermis, present elsewhere on the stem. These communicate with the intercellular spaces of the medullary rays, arranged radially so that gaseous exchange can take place between the air outside and the living cells inside the stem.

Ferns

Gaseous exchange is carried out in the fronds of ferns through the stomata, which are like those of a flowering plant. The spore-bearing generation has true roots possessing root hairs which function also like those of a flowering plant. The surface of the prothallus is not gas-proof, allowing gaseous exchange to occur rapidly all over it, and its rhizoids function like root hairs.

Thallophyta

These plants have so large an external surface compared with their volume that there is ample provision for sufficient gaseous exchange through their surfaces, and no special organs, e.g. stomata and lenticels, or internal air spaces, are found. Their external cell walls readily allow diffusion of gases through them.

Experiment 20—To find out whether Genrrinating Seeds absorb Oxygen

Two sets of apparatus are set up, set A containing no seeds to act as a control, and set B containing germinating seeds. The caustic soda will absorb any carbon dioxide which is set free by the peas. Any rise of liquid in the delivery tube will show whether any gas has been removed from the air in the flask. When the experiment has been going for some time, as indicated by the peas no longer developing, the bung is carefully removed and a lighted taper put in to show whether the remaining gas contains oxygen or not.

Experiment 21— To show that all Parts of Plants produce Carbon Dioxide, and to find out whether these Parts use up Oxygen

This one is to act as a control to show if the lime water appears milky, and in each case whether it is due to the presence of a part of a plant or not. The following are suggested : dandelion flowers in water, leaves in water, stems such as twigs of gorse in water, a bud of Brussels sprout, a tuber of potato, a crocus conn, an onion bulb, a rhizome of Solomon ’s seal, and a tap-root of carrot. Those parts which are normally under ground are best kept slightly moist in damp blotting-paper. The jar containing the green parts of the plant should be placed in the dark, so that no effect can be produced by light. The best results are obtained by standing each apparatus in a warm place. When the skin which appears on the lime water in the apparatus does not appear to be getting any thicker, the stopper is removed from the bell jar and a lighted taper put in to see whether the part of the plant inside has used up the available oxygen.

Experiment 22—To show that Roots give off Carbon Dioxide

A bottle is half-filled with distilled water and shaken for some time to dissolve oxygen. The soil is washed from the roots of a groundsel plant in running water. The solution is tinted with Congo red, which is extremely sensitive to the presence of carbon dioxide, and some is poured into a test-tube, nearly filling it. The plant is placed with its roots in it and held in place by a wad of cotton wool. In order to make certain that the colour change is not due to carbon dioxide diffusing through the cotton wool, a control tube is set up exactly like the tube under test, but containing no plant. If set up at the beginning of a lesson the colour change from red to purple should be apparent within half an hour.

This experiment can be done with healthy Spirogyra if it is washed in running water before being put in the test-tube, which is, after inserting the cotton wool, put in the dark.

Experiment 23—To show that Germinating Seeds produce Heat Energy

Two thermos flasks are used for this experiment. In one is placed a good handful of previously soaked wheat or barley grains. In the other is placed a similar quantity of grains which have been killed by boiling in water and then soaked overnight in formalin solution to prevent subsequent decay and production of heat by micro-organisms feeding on them.

A Fahrenheit thermometer is placed in each with the bulb immersed in the grains and plugs of cotton wool are placed in the necks.

The initial readings of both thermometers are recorded and also a daily reading for a period of one week. The thermometer in the first flask will soon begin to record a rise of temperature compared with that in the second, showing the production of heat energy by the germinating seeds.

The purpose of the second flask is to act as a control and to ensure that the rise of temperature in the first flask is not due to external change of temperature. In the latter case both flasks would be affected equally and both thermometers would indicate a similar rise in temperature.

Experiment 24—To show that Germinating Seeds lose Part of their Dry Weight by Respiration

Two basins are weighed and an equal weight of dry barley or wheat grains is placed in each.

The grains in one basin are moistened and left until the grains have sprouted. Both basins are then dried in a steam oven for an hour, cooled and weighed when cold. This process is repeated until no further loss in weight is recorded.

In both cases a loss in weight is found. The percentage loss in weight of the germinated grains will be found to be greater than that of the ungerminated grains, showing loss of dry matter by respiration.

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