for their use. The "loamy soil" which a farmer loves contains from forty to seventy per cent of sand.

Here, again, we see the great usefulness of earthworms. In sand they, like the roots, can make their way so easily that they have little need to remove the soil by swallowing it, the only means at their disposal. But in a stiffer soil they are obliged to do this, and thus they let in both air and water, to the great advantage of the plants, while they also spare the roots much labor by preparing for them airy passages, down which they can run with ease.

But though roots take advantage of these ready-made channels, and are evidently all the better for them, they do not let go their hold on the soil, but keep a close grasp of it, lining the worm-burrows with thread-like fibers, which cling fast to the sides.

Roots coming in contact with a piece of limestone will leave upon it a perfect impression of themselves, even to the hairs with which they are fringed, showing how, like the lichens, they have eaten their way into the solid substance.

How do they do it? We can hardly do more than conjecture; but it seems probable that the acid in the roots acts much as acid contained in a bladder would.

If

glass tube is filled with water made slightly acid with vinegar, and then covered with a piece of moistened bladder strained tightly over the mouth, and in contact with the liquid, this will represent the root, though the resemblance would, of course, be closer if the tube itself were of bladder. This, however, seems to be the only practicable way of trying the experiment. The acid is very weak, as the acid in the roots is weak; but if salts, such as phosphate of lime, and others found in the soil,

are now strewn upon the bladder, they will in a short time begin to pass through it into the tube, being dissolved by the weak acid in its pores.

The acid in the roots acts, it is supposed, in a similar way, and thus the dissolved minerals are sucked in. But as before said, living things have more power than dead ones; so it may well be that roots, like lichens, dissolve more than the weak acid alone would do.

The roots take up what they themselves dissolve from the particles of soil immediately surrounding and closely touching them, and also what the water in the soil has dissolved for them, with the help of carbon dioxide and other gases.

The water thus taken up-for what is dissolved by the roots and what is dissolved by water and gas are taken up together the water thus taken up is a very weak solution of various salts-phosphates and others so weak that it may fairly be compared with ordinary drinking-water.

No water in nature is or can be perfectly pure, as has been said, because it is constantly dissolving something wherever it goes. And though even with what the roots have dissolved the solution is still so weak as to pass for ordinary water, yet it must be borne in mind that the roots are constantly sucking it in, and that the leaves are as constantly returning the water to the air-only the water, however. The salts remain behind and accumulate day by day.

The same sort of thing on a vast scale goes on with the rivers and the ocean. River water is generally tasteless, though it, too, contains various salts dissolved in it. This small proportion of salts is, however, being constantly poured into the ocean, while the sun is constantly

The

taking away by evaporation almost pure water. salts, therefore, accumulate, and sea water is salt and bitter in consequence.

The salts left in a plant do not usually make it salt or bitter, because the quantity is altogether extremely minute in proportion to the plant's size; and as they are distributed through the whole of its substance, there is a continual demand for them while the plant is growing or putting forth fresh leaves.

But if a plant is stunted by drought it may become actually bitter. A cabbage, for instance, which has not reached perhaps a quarter its proper size for want of water, will be quite bitter; and the reason seems to be that the salts, which would have been enough for a large cabbage, are compressed into a very small one. Or, in other words, the roots have not been able to find enough water to dilute the food which they have gathered, as well as to keep pace with the transpiration of the leaves, and to allow of their proper growth.

QUESTIONS FOR REVIEW

I. Show how soils vary in their ability to suck moisture from the air or to keep it.

2. How do soils differ in their readiness to give up moisture? Illustrate.

3. How is the surface soil moistened in time of drought? 4. How do the roots vary in different kinds of plants?

5. Describe the character of a root.

6. Why is it necessary that soil should be both fine and well mixed?

7. Why is clay a poor soil for plants?

8. What is the character of the water taken up by the roots? 9. Why is the sea salt?

CHAPTER XI

FOOD FROM THE SOIL

It is but a very small part of their food after all which plants, generally speaking, draw from the mineral matter of the soil in which they grow; and yet this small quantity is not merely important, but absolutely necessary. It is dissolved by water and gases, and by the action of the plant itself, and is then taken up by the roots, especially the younger, finer roots, and root-hairs, by which it is passed on to the stem, and so is conveyed to every part, not only to branches, leaves, and buds, but also to flowers and fruit. Every part of a plant needs some amount of mineral matter, and the plant cannot obtain it without water, for whether dissolved by the plant's roots or otherwise, it is in each case taken up in very diluted condition; so diluted, indeed, that the water containing it is hardly to be distinguished from ordinary drinking-water.

The plant could not be sufficiently nourished by these very weak dilutions, especially while it is growing, but for the fact that it is constantly receiving them.

Perhaps one of the most striking examples of the way in which plants are fed by this very weak food is to be found among the sea-weeds. Many sea-weeds contain large quantities of iodine, which, like the rest of their food, they draw from the sea. With the smell of iodine we are all no doubt familiar; but if we mix one part of iodine with three hundred thousand parts of water we

entirely lose it; that is to say, no one of our senses is keen enough to detect it. We can neither see, nor taste, nor smell it. But of course it is there, and we can find it again by adding starch, which is turned to a brilliant blue by coming in contact with even this minute quantity.

extract it.

But the iodine contained in sea-water is less even than this it is less even than the hundredth part of this infinitesimal amount. And yet the sea-weed manages to And although plants take their mineral food in such weak dilutions that we cannot detect its presence either by taste or smell, and might be inclined to think that it can matter very little what it is, yet they are discriminating; and their roots have to some extent the power of choosing what they will or will not take up.

This is evident from the fact that plants growing side by side will take up different food, or take it in very different proportions.

There is, for instance, the common reed and the common species of moss, which both grow in bogs. The soil is dissolved by water and gases equally for both, and both take up a good deal of dissolved flint, or silica; but the reed takes up also a very small quantity of salt, a little more, but still a very small quantity, of iron, no soda, a little magnesia, and a great deal of phosphoric acid; whereas the moss, which grows close by, takes very little either of phosphoric acid, magnesia, or salt, but some soda, and much iron.

The same thing is also true of the farmer's crops, and it is for this reason that he varies them, not growing the same crop year after year, or even two years running, on the same soil, lest it should be exhausted and unable to feed them.