Plants must have air to breathe, both above and below the soil, and the microscope is showing us here how a sandy loam allows the air to reach the roots.
In mountain regions these "storm ploughs," as we may call them, not only help to renew and prepare the soil in the valleys, but are a part of the machinery of delivery of new soil from mountain to valley. When trees on the mountainside are overturned, they not only bring up the soil, which the mountain rains quickly carry to the valleys, but the roots having penetrated—as they always do—into the crevices of the rocks, bring up stones already partly decayed by the acids of the roots. These stones, as the roots die, decay and so release their hold, and also go tumbling down toward the valley.
Consider how much of this storm-ploughing must be done in the forests of the world in a single year, and that this has been going on ever since trees grew big on the face of the earth. In a storm in the woods of California, Muir heard trees falling at the rate of one every two or three minutes. And, as I said, it is precisely the trees that can do the most ploughing—the older and larger trees—that are most apt to go down before the wind. Younger trees will bend while older and stiffer trees hold on to the last. Before a mountain gale, pines, six feet in diameter, will bend like grass. But when the roots, long and strong as they are, can no longer resist the prying of the mighty lever—the trunk with its limbs and branches—swaying in the winds, down go the old giants with crashes that shake the hills. After a violent gale the ground is covered thick with fallen trunks[7] that lie crossed like storm-lodged wheat.
There are two trees, however, Muir says, that are never blown down so long as they continue in good health. These are the juniper and dwarf pine of the summit peaks.
"Their stout, crooked roots grip the storm-beaten ledges like eagle's claws, while their lithe, cord-like branches bend round completely, offering but slight holds for winds, however violent."
AT THE STORM FESTIVAL WITH MR. MUIR
Trees were among Muir's best friends, and he spent a large part of his life chumming with them. What do you think that man did once? He was always doing such things. He climbed a tree in a terrific gale so that he could see right into the heart of the storm and watch everything that was going on. Just hear him tell about it:
"After cautiously casting about I made choice of the tallest of a group of Douglas spruces that were growing close together like a tuft of grass, no one of which seemed likely to fall unless the rest fell with it. Being accustomed to climb trees in making botanical studies, I experienced no difficulty in reaching the top of this one, and never before did I enjoy so noble an exhilaration of motion."
And such odors! These winds had come all the way from the sea, over beds of flowers in the mountain meadows of the Sierras; then across the plains and up the foot-hills and into the piny woods "with all the varied incense gathered by the way."