CHAPTER II.
HOW A GREEN PLANT FEEDS.

5. THE FOOD WHICH A GREEN PLANT
OBTAINS FROM THE SOIL.

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),1gram.
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,1litre.