Читать книгу An Introduction to Nature-study - E. Stenhouse - Страница 10

1. A plant cannot grow permanently in damp sawdust or clean sand.—Notice that the seedlings which were grown in damp sawdust presently wither and die, while those which were grown in soil flourish, and, with proper care, come to maturity. Obtain some clean sand, and, to be sure that there is nothing in it which water can dissolve, wash the sand in several changes of clean water. Germinate some seeds in the sand, keeping it damp. The resulting plants in this case also wither and die. Evidently soil contains some plant-food which the plant cannot obtain from sawdust or clean sand. What is this food?

Fig. 18.—Porcelain crucible heated by Bunsen burner.

2. The amount of water and mineral matter in plants.—Take a healthy plant, say a bean plant, and weigh it. Then dry it thoroughly in the oven and weigh it again. It will be found very much lighter; the difference in weight represents the water which has been driven off. Burn the dried plant. When the flame goes out notice the black charcoal which is obtained. Continue the heating and observe that at last nothing is left but a little grey ash. This experiment can be performed over an ordinary fire by using an old shovel or a tile, but if you can use a porcelain crucible (without lid) and a Bunsen burner (Fig. 18) you will get better results. If a chemical balance is available, weigh the ash and compare it with the weight of ash obtained from an ordinary bean seed, such as that which gave rise to the plant you have used.

The ash from the plant is much greater than that got from the seed. This extra ash must have been taken from the soil during the plant’s growth.

3. A nutritive solution.—Make, or ask a dispensing chemist to make, the following solution:

Potassium nitrate
(consists of potassium, nitrogen, and oxygen),	1	gram.
Sodium chloride
(consists of sodium and chlorine),	½	”
Calcium sulphate
(consists of calcium, sulphur, and oxygen),	½	”
Magnesium sulphate
(consists of magnesium, sulphur, and oxygen),	½	”
Calcium phosphate
(consists of calcium, phosphorus, and oxygen),	½	”
Water,	1	litre.

(A few drops of a dilute solution of sulphate, or chloride, of iron should be added.)

Water, with this solution, a plant growing in wet sand, and when it is well grown, dry and burn it. As much ash is obtained as from a plant of the same size grown in soil. Notice the difference between such a plant and one which has had water only supplied to it.

4. Water culture.—Fix two similar young plants in corks as shown in Fig. 19, and put the corks into two bottles, the first of which contains pure water and the second the nutritive solution, and let the roots of the seedlings dip into the liquids. Cover the outsides of the bottles with rolls of paper to keep out the light. Notice that the plant living in the nutritive solution thrives, while the other presently withers. Dry and burn the former, and observe that it yields more ash than does a seed such as that from which it sprang.

5. Plants obtain their mineral food from the soil by their roots.—As the roots are the only parts of the plant which are in contact with the nutritive solution, or which (under ordinary conditions) are in the soil, the mineral matter must be taken in by the roots.

6. The root-hairs.—Take up a seedling which has been growing in damp sand, and observe the small particles of sand adhering to the root-hairs (p. 17). The hairs of a plant’s root and rootlets apply themselves very closely to particles of soil (Fig. 20), and the mineral food (dissolved in water) passes into the hairs and so gets into the root and thence to the other parts of the plant.

7. Roots as storehouses of food.—Examine, before the plants flower, the roots of a turnip, a carrot, and a radish, and notice how greatly they are swollen. You know that these roots are valued as foods; of what use do you think the stored food is to the plants themselves?

The food of a young seedling.—When such a seed as that of a bean is germinated in damp sawdust or wet clean sand, and kept in a warm and light place, it puts out a radicle, which grows downwards and becomes the main root, and a stem which grows upwards and bears green leaves. After a time the main root branches, giving off side roots, which spread in all directions through the sawdust or sand. The main root and the rootlets bear very fine fluffy hairs for a short length, which is situated just behind their points (Fig. 11), and these root hairs come into very close contact with particles of damp sawdust or sand (Fig. 20), the moisture passing into them and thus reaching the main root, from which it is distributed to the various parts of the plant. The stem likewise flourishes, growing in length and thickness, and putting out new leaves.

All this time the young bean plant is living on the food material stored up in its cotyledons (p. 6); and if the sand or sawdust is kept moist, with even pure water, this seed food is at first quite sufficient. When at last the seed food is all used up, however, and all that remains of the cotyledons is a shrivelled skin, the plant begins to droop and wither from lack of food.

Plants obtain food from the soil.—Contrast this with the condition of a seedling which has been grown in soil. It still flourishes, even when the seed food is used up, for it is drawing up food from the soil—food which could not be obtained from the damp sawdust or clean sand.

That the plant really has taken up some solid matter from the soil can be proved by a few simple experiments. A plant which has been growing in soil for some time after its seed food is used up is dried and burnt, and the ashes are weighed. The weight of ash or mineral matter thus obtained is found to be considerably greater than that of the ash obtained from an ungerminated seed, or from a seedling grown in damp sawdust or sand which has only been supplied with pure water.

The mineral food of plants.—The composition of the ash obtained from various plants has been carefully determined by chemists, and in this manner they have been able to find out what substances must be present in soil in order that the plant may obtain all the mineral food it requires. A mixture of potassium nitrate (nitre), sodium chloride (common salt), calcium sulphate (plaster of Paris), magnesium sulphate (Epsom salts), calcium phosphate, and chloride (or sulphate) of iron—dissolved in water in the proportions specified on p. 27—has been found to supply the necessary elements of the mineral food in a form which the plant can readily use. That such a mixture is capable of supporting the plant, while water alone is incapable of doing so, may be seen by growing a plant—in the manner shown in Fig. 19—in this solution. If, in addition, the plant is supplied with light and fresh air, it will grow in a perfectly healthy and normal manner. If any of the constituents (except the common salt) are omitted, the plant will suffer. On the other hand, a plant which is growing in pure water will presently die, from the lack of the necessary mineral food.

Fig. 19.—Plant growing in a nutritive solution

of salts. The bottle should be covered with a

roll of paper to keep out the light.

The work of the roots.—These experiments show that water—of which a large proportion of a plant consists—and the mineral constituents of its food (dissolved in the soil-water) are taken up from the soil by the roots. In ordinary soil the rootlets spread out on all sides, dividing and subdividing, seeking for this very weak solution of mineral salts. Even when soil appears practically dry, a very thin film of moisture covers each little particle of earth, and the root hairs become closely applied to these little particles (Fig. 20), so that the water passes through their walls and gradually makes its way to the main root, the stem, and the leaves.

Fig. 20.—Tip of a root hair with adhering particles of soil. (×240.)

Roots sometimes perform other duties in addition to those of fixing the plant in the soil and providing it with water and mineral food. It is usual, for example, for biennial plants—which produce flowers and seeds in their second year, and then die—to take in much more food during their first season than they require at the time, and to store up the surplus in readiness for the great effort of the second year. These reserve materials are often stored in the roots, which then become swollen and fleshy, like those of the turnip, radish, and carrot.