You can keep seedlings growing quite well, however, if you place them in glass jars so that their roots are in water, or even in closed glass jars standing over water, so that the air is thoroughly moist. You will then be able to see very well numbers of fine white hairs on the roots (see fig. 8). These hairs are very important and absorb the water which keeps the whole plant moist.
Fig. 8. Maize seedlings growing enclosed in damp air, supported on a wire stand over dish of water so that their roots do not touch it, but grow in the air. Notice the “root hairs” growing out from the roots.
You have now seen that seedlings require water for their life just as animals do; and also that young plants are provided by their parents with a store of food which is largely starch, and which they use up during their early growth.
CHAPTER IV.
FOOD MATERIALS OF THE OLDER PLANT
(1) IN THE SOIL
As we have just seen, young seedlings are supplied with stores of food, starch, and other things, which are packed in their cotyledons and are used up by them as they grow. But we also saw that as the plant gets older these stores get emptier, and finally the nurse leaves shrivel up entirely when their contents are exhausted. All the same, however, the plant continues to grow. Surely it cannot do this on nothing, any more than an animal could? When the young calves cease to be fed with milk, their food changes, and they begin to eat grass; this gives them individually more work, for grass is not a “prepared food” like milk. Very much the same thing happens with seedlings. Their prepared food supply gets used up, and they must find food for themselves. Where do they find it?
When you remember the fine hairs on the young parts of roots which absorb water from the soil or sawdust, it is quite natural to think at once of the soil as a possible place for them to find their food; and, indeed, this is partly the case. The water in the soil is not perfectly pure, for there are many different “salts” dissolved in it. By “salts” one does not mean only table salt, but also any kind of mineral in solution, such as salts of iron or portions of chalk or limestone, or even some of the minerals which make up granite. These may all be dissolved in rain-water just as sugar is dissolved in your tea, and so spread equally through it. As the water enters the roots of plants through the hairs, these dissolved salts come in with it, and so get distributed over the whole plant. The root hairs cannot “eat” particles of soil, but they twine in among the fine grains and absorb the little films of water which cling to them.