THE NUTRITION OF NON-GREEN PLANTS: THE SAPROPHYTIC MODE OF NUTRITION

PLANTS such as Mucor and the mushroom, which do not possess chlorophyll, must therefore feed differently from green plants. Lacking chlorophyll, photosynthesis cannot be carried out, I.e. carbohydrates cannot be made from raw materials, carbon dioxide is of no value, nor is light, and also the food required must not only be ready made but in fluid form, since no plants are capable of taking in solid particles ; therefore the food supply which forms the plants ’ habitat must be moist. Since they are not dependent upon light they can grow at any time, but as a rise in temperature greatly speeds up chemical changes as well as the rate of diffusion, they grow best under natural conditions just after dawn and sunset.

The absorption of material from dead ready-made foodstuffs is termed the saprophytic mode of feeding, since the food is generally the dead bodies of plants and animals upon which the fungus lives and feeds, causing them to rot. The mycelium of the fungus produces enzymes which pass in solution through the membrane enclosing the protoplasm into the surrounding moisture. The ready-made food is digested by these enzymes, such digestion occurring outside the organism being called extracellular. The digested food in solution is absorbed by the mycelium, and consists largely of sugars and amino-acids and simple peptones. From these the plant builds up its own peculiar protoplasm in the usual way, and stores reserves in the form of oils and glycogen. The membrane enclosing the protoplasm is termed fungal cellulose.

Experiment 48—To find out under what Conditions a Saprophytic Fungus such as Mucor groivs best

Four sterilized Petri dishes are set up as follows and covered by deep covers to allow free growth of Mucor, but to keep out other saprophytes floating as spores in the air:—

A, containing dry toast, I.e. dry sterile food.

B, containing wet toast, I.e. moist sterile food.

C, containing wet bread, I.e. moist non-sterile food.

D, containing ordinary bread, I.e. bread as normally exposed to fungi.

The purpose of toasting is to sterilize the food ; by comparing the results of B and C it can be determined whether toasting destroys the food available.

The food in each dish is liberally infected with a blob of spores from a Mucor colony and left until obvious results appear. By putting a fifth dish like B in a place warmer than the others the effect of a warmer environment is readily obtained, and affords one of the best methods of demonstrating the effect of rise of temperature upon chemical biological activity.

Many bacteria also live saprophytically, some being of great biological importance for they cause organic materials to rot and decompose, producing simple soluble substances of value to green plants. Such organisms are termed bacteria of decay. Among the useful substances produced by their activity are carbon dioxide, ammonia and nitrates.

Experiment 49—To show the Production of Carbon Dioxide by Bacteria in Soil

Two tubes are set up, the muslin bag in A containing moist sterilized soil, and in B, containing ordinary moist soil. They are left for several days. The lime water in A remains clear, while that in B turns cloudy.

Experiment 50—To show that Bacteria of Decay set free Ammonia and form Nitrates

Milk is sterilized by being boiled in a beaker for ten minutes on two sucoessive days, being kept well covered all the time with a Petri dish lid.

Two sterilized tubes sealed with sterilized cotton-wool are set up :—

A, containing sterilized milk.

B, containing ordinary milk.

Two sterilized Petri dishes are also set up and covered with deep covers to allow acoess to oxygen :—

C, containing sterilized milk and moist soil.

D, containing sterilized milk and moist soil mixed with powdered chalk.

Nessler ’s solution gives a brown coloration in the presence of ammonia.

A solution of diphenylamine gives a dark-blue colour with nitrates.

The results of this experiment will show that soil contains putrefying bacteria which produce ammonia from proteins ; nitrifying bacteria which, in the presence of calcium carbonate, change ammonia to nitrates.

Fermentation

This was the term originally applied to the action of yeast on sugary solutions. It is often used to describe the action of an organism on a food substance with the product of gaseous products. Yeast lives saprophytically on sugar-containing solutions. Such natural ones as grape juice or malt contain, in addition to sugar, various mineral salts. The yeast cells absorb the sugar and the salts and use them for food. Yeast is able to synthesize proteins from sugar and relatively simple compounds of nitrogen, e.g. ammonium tartrate, which occur in fruit juices, using the energy obtained by respiration.

Normally yeast respires anaerobically, converting simple sugars to alcohol and carbon dioxide to release energy. This change is effected by means of the enzyme zymase :—

C6H1206 = 2C2H5OH + 2C02 + energy.

Sugar alcohol

Experiment 51—To show the Production of Carbon Dioxide by Yeast

Two teaspoonfuls of glucose are dissolved in 50 c.c. of water. A small quantity of baker ’s yeast is added and well mixed with the sugar solution. This liquid is then poured into a flask fitted with a cork carrying a delivery tube with its outer end dipping into a test-tube of lime water. The flask should be kept warm, and after an hour or so the liquid will be seen to bubble, and as the bubbles of gas escape through the lime water the latter turns milky showing the gas to be carbon dioxide.

The enzyme zymase was the first one to be isolated from a living organism. This was effected by Biichner, who ground up yeast with a flinty earth to crush the cells and liberate their contents. He then filtered the liquid and found that it would ferment sugar solutions.

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