From these experiments you see that it is not the soil which is necessary to the plants, but that certain salts in solution in the water held by the soil particles are very important. When all the salts are present in the water, as was the case in jar C, the plant can grow just as well as one in the soil; but when it has not these salts it must die. The salts in solution, therefore, must be a very important part of the food. Are they the only food the plant gets?
CHAPTER V.
FOOD MATERIALS OF THE OLDER PLANT
(2) IN THE AIR
The experiments you have just done show that plants absolutely require the mineral salts dissolved in the water of the soil or of their food solutions. Yet although these salts are so necessary, they do not use a large quantity of them, as you may prove by taking the solution C, which is left after the plant has grown in it, and slowly drying off all the water (taking care not to destroy a part of the salt crystals) by gentle heat, and then weighing this dry salt, and comparing its weight with that of the salts you put into C. You will find that the growing plant has only removed a small quantity of the salts. Yet the plant should have grown to some considerable size. Of course, the water itself goes into the plant tissues, but you can drive this off by gentle heat. Before drying it, however, cut off a part of the plant which is equal in weight to the weight of the young plant you put into the food solution at first (see p. [16]), so that you have only to deal with the amount of its growth while using the food solution. Then if you weigh the fully dried plant, you get the weight of the solid structure added to its body while it was growing in the food solution, and you will find that this is much heavier than the amount of the salts it used during its growth.
What is this extra substance?
Now let us examine the dried plant more carefully. Heat it on an open dish, and you will find that it goes black and chars, very like the charred wood on a fire or specially prepared charcoal. The black charcoal is well known to consist chiefly of carbon, and so does this black plant-ash. You know that charcoal can burn, and so will this charred plant if you heat it more strongly. Although you can burn the carbon (that is, you can make it combine with oxygen gas and go off in an invisible form), yet you cannot absolutely destroy it. Like all elements it is not to be made or destroyed by us, nor can the plant make carbon for itself.
If you examine the list of substances you put into the food solution once more, you will find that carbon is not among them, nor is it contained in any of them.
Carbon, then, is the extra substance which makes the weight of the plant greater than that of the salts used from its food solution.
Where does the plant find this carbon?
You may know that there are three chief gases in the air: oxygen and nitrogen, which are the important parts for our breathing, and a little carbonic acid gas, which you may remember is breathed out by animals and plants (see p. [6]), and is made of carbon joined with oxygen. As there was no carbon in the food solution, and the plant was surrounded by air containing carbon and oxygen in the invisible form of gas, the idea is suggested that perhaps it is from the air that the plant gets its carbon. Now let us see if this is true by trying the effect of removing the carbonic acid gas from the air in which the plant is growing.