You will find that the gas bubbles are given off much more rapidly when the plant is placed in bright sunshine than when it is shaded, and that when the plant is in darkness the bubbles stop altogether. This seems to show us that the sunshine must assist the plant to split up the carbonic acid gas, and we will find out more about this later on (see p. [25]).
We have now found that carbon forms a large part of the plant body, that plants cannot grow in air in which there is no carbonic acid gas, and that in getting the carbon from the carbonic acid gas, they split it up and give off the oxygen. So that we see that plants use the carbon in the air as well as the salts dissolved in the water of the soil as raw materials, with which they finally build up their food. We must now try to find what food substance it is that they build up from these raw materials.
CHAPTER VI.
THE FOOD MANUFACTURED BY THE PLANT
You will remember that much of the food provided in the nurse leaves consisted of starch, and that the baby plants use this food as they grow.
In the full grown plant we also find much starch; in fact, nearly all the parts of plants which we eat as food contain large quantities of starch, as you can test with iodine in potatoes, turnips, radishes, oatmeal, flour, and a host of our other vegetable foods. This is also the case in many parts of plants which we do not generally use as food, for example, in the lily and tulip bulbs, underground stems of Solomon’s Seal, and the stems and leaves of most plants. So that we find that the food grains of starch are developed in grown plants, and are not only provided for the young ones.
What is starch made of? Try heating a piece of laundry starch on an iron plate or the bars of the grate, and you will see that it blackens, and finally, if you put a light to it, may burn. If you simply heat it without quite burning it, you will find that it chars and goes black like a piece of charcoal. The solid element of starch is carbon. Now you may remember that in the plant growing under the bell-jar from which we shut out all the carbonic acid gas, we found that the leaves did not show any starch (see p. [20]). The plant had not been able to build up starch without the carbon obtained from the air.
The leaves of a plant are spread out in the sunshine and air, and it is in the leaves that we get the starch first formed. The leaves, in fact, are the food factories of the plant. You should study the appearance of starch in the leaves. As their green colour hides the iodine colouration, it is better first to remove it from any leaves you are studying in the following manner. So soon after picking them as possible, throw them on to some very hot or boiling water for a moment. This kills them quickly and makes them soft; then put them in a jar or tube of alcohol,[4] and leave them in it overnight. By next day the green colour should be gone, having been absorbed out of the tissues by the alcohol, and the leaves left yellowish or white. Then put them to soak in water till the stiffness caused by the alcohol has gone, when you should add the iodine. If you examine ordinary leaves in this way you will find that they go violet or brownish blue, showing that they contain starch.
Now do leaves always contain starch? You will remember that the oxygen bubbles were given off much more quickly from the plants in the sunlight than from those in the dark (see p. [21]). This shows that the leaves in the sunlight split up the carbonic acid gas more quickly than the others, which would give them more carbon to work on, and therefore it seems that they should be able to build up more starch in the light than in dull weather or darkness. You can see if this is true by doing a simple experiment.