THE NUTRITION OF GREEN PLANTS: THE HOLOPHYTIC MODE OF NUTRITION

The way in which green plants feed is peculiar to them in that the substances obtained not only are in fluid form but because these substances are of no value for supplying energy, and therefore cannot be regarded as food. We shall therefore throughout this article refer to the substances absorbed by green plants for their particular nutrition as raw materials, since from them the plant makes its own food.

The raw materials are water, the gas carbon dioxide and mineral salts, particularly nitrates, sulphates and phosphates. Water enters the organism by osmosis, whilst the salts enter by diffusion from soil water or the water environment.

Aquatic plants obtain their C02 in solution also by liquid diffusion, whereas in land plants it enters by gaseous diffusion. We can define this mode of nutrition, termed holophytic plant, as the mode of feeding upon raw materials in a fluid condition by diffusion.

The green colour of such plants is due to the presence of chlorophyll, a mixture of four pigments, blue-green chlorophyll a, pure green chlorophyll fi, orange carotin and yellow xanthophyll. Plants lacking iron or oxygen or light become a sickly yellow, for under such conditions yellow pigments only are made instead of chlorophyll. No pigment in chlorophyll is an iron compound, so that the iron must act as a catalyst during chlorophyll formation. Magnesium salts are also essential since chlorophyll a and /? are magnesium compounds.

Chlorophyll absorbs energy from sunlight, particularly red, orange, indigo and violet rays. Red and orange rays are of high energy content, whereas the violet rays are chemically active, I.e. they can cause some chemical changes to occur more readily.

Experiment 36—To show the Absorption of Certain Rays of Light by Chlorophyll A spectroscope is set up and the spectrum of white light is viewed. A glass box containing chlorophyll solution is placed between the source of light and the slit, when parts of the spectrum will be seen to be missing. A chlorophyll solution is readily made by boiling young leaves in industrial alcohol until the liquid is a deep green. The solution should be freshly made, as after a time it becomes turbid.

In the presence of light and of carbon dioxide the green parts of plants make sugar, much of which is then converted into starch. When a gram of starch is completely oxidized to carbon dioxide and water, 4-1 kilocalories of heat are set free. Therefore in order to build up starch from these raw materials this amount of energy is required, being absorbed by chlorophyll from sunlight. During the making of sugar, oxygen is set free as a by-product.

Experiment 37—To show that Starch is only produced in Green Plants during Sunlight

Two pot plants are put in an airy dark cupboard twenty-four hours before the experiment in order to ’destarch ’the leaves. One is then exposed to sunlight for some hours. A similar leaf from each is boiled in water, decolorized in methylated spirits and then placed in hot water. They are then immersed in dilute aqueous iodine for ten minutes. A brownish tint is produced by cellulose with iodine, so that instead of the characteristic blue due to starch, the colour where starch is present may be greenish black.

Stencils may be placed over leaves of plants in the evening, to be treated and tested as in at the end of the next day.

Spirogyra may be treated as in and then examined microscopically to show starch around each of the pyrenoids, although in this case cold alcohol must be used for decolorizing, otherwise the cell contents collapse. A very beautiful experiment to show that only certain kinds of light are utilized by green plants is that of placing a destarched plant in a light-tight box, and arranging the light which enters through a slit at the side to pass through a prism so that a spectrum falls on a large leaf. On decolorizing and testing with iodine the areas of the leaf which have produced starch can be found and compared with the areas upon which red and violet light has fallen.

Experiment 38—To show the Formation of Starch in the Green Parts only of Leaves

The variegated leaves of geranium, of Japanese maple or of nasturtium, which have been exposed to sunlight, are wetted and so made to adhere to the polished side of a gas jar cover.

A map, both of the outline as well as the chlorophyll area, is drawn in pencil on the ground-glass side. The leaf is then treated as in Rxpt. 37. When limp it is again spread upon the plate and the iodine map compared with the chlorophyll map.

Experiment 39—To show that Starch is not produced unless Carbon Dioxide is Present

Destarched mint leaves in a small bottle of water are placed under a large bell jar on a vaselined glass plate. A candle is burned inside the bell jar in order to remove oxygen and replace it by carbon dioxide. A similar apparatus is set up containing in place of the candle a dish of caustic soda to absorb any carbon dioxide.

Both sets are exposed to strong sunlight, the tops of the bell jars being covered by large ’dunce-cap ’paper covers to prevent the curved sides of the bell jars from acting as lenses and so burning the leaves. After two or three hours ’ exposure leaves from each bell jar are killed, decolorized and tested with iodine. Starch is found to be present only in the leaves from the first bell jar.

Experiment 40—To show the Liberation of Oxygen front Green Plants only in the Presence of Sunlight and of Carbon Dioxide

Three sets of apparatus are set up as in the diagram. In A the liquid is distilled water. In B and C liquid is distilled water saturated with carbon dioxide. In each flask some sprigs of mint or plants of Elodea are placed.

A and B are exposed to strong sunlight until sufficient gas has collected in one tube to make a test for oxygen with a glowing splint, whilst C is kept in darkness. The results are tabulated below.

It is usually necessary to joggle the apparatus in order to dislodge any gas adhering to the leaves.

Quite good results are obtainable with Spirogyra instead of mint or Elodea, although the tiny bubbles of gas produced are rather difficult to dislodge.

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