Fig. 16. Striped leaves; the white stripes show no starch when stained with iodine.
One of the great contrasts between the leaves in the air, and the parts of the plant underground, is that the leaves are bright green in colour, and the underground parts are yellowish or brown. It has been found that the green colour in leaves is very important in the building up of the starch. You can see this in the case of leaves which have parts quite colourless, as in those which are variegated or striped. Take the leaves of such a plant, which have been exposed to a good light, and test them in the usual way for starch. You will find that the pale stripes of the leaf show no colour with iodine, because they are empty of starch, owing to the fact that the green colour was not there to build it up. The value of the green colour is that it absorbs the energy of the sunlight, and uses it to get the carbon from the carbonic acid gas, and then to build the carbon into starch.
Now you will remember in doing the experiments on the food solutions (see p. [17]), that one of the plants lost its green colour, turned yellowish, and finally died. That was the plant which had no iron in its food solution. We have found, therefore, that without iron a plant cannot build up its green colour, and without its green colour it cannot use the store of carbon in the air to build up its food. This is only one example of the importance of mineral salts to the plant. Salts containing nitrogen are equally vital, while a number of mineral compounds are necessary for healthy growth. So that we see that the minerals absorbed in solution by the roots, as well as the carbonic acid gas absorbed from the air by the leaves, and the energy of light absorbed by the green colour are all equally necessary to the life of the plant, as all help in the building up of its food.
We have now seen that plants require food just as much as animals do; but that they use different and simpler elements from which they build it up for themselves, unlike the animals, which require their starchy foods to be ready built up for them. The foods which plants make they use in growing, and the other activities of their lives, just as animals do.
CHAPTER VII.
THE CIRCULATION OF WATER
As we have already found out, water is one of the things which are necessary for the well-being of plants. Seedlings can begin to sprout only when they are well supplied with it, and in the growing plant it is the water in the cells which keeps it firm and fresh. Directly the plant is deprived of some of its water it becomes limp and flabby, and “withers.” We noticed in Chapter IV. that the rootlets absorb the water (with its salts contained in solution) from the soil, and from them it travels all over the plant. The salts dissolved in water, however, are in very weak solution, and to provide the plant with sufficient of them for its growth it is necessary that a continuous stream of water should enter the plant. How is this stream kept up?
