In all these plant republics each citizen must pay something into the common treasury for its board and keep. This fund not only meets "national expenses" during the lifetime of the ones who pay these taxes, but it helps prepare the land for the great citizens of the future—the trees. In another hundred years—making two hundred in all, after the arrival of the very first lichens—low shrubs and bushes often find spots in these new communities where the soil is thick enough for their needs.
It is very curious how members of the plant world, growing side by side, seek their food at different depths, and send out their roots accordingly. It reminds one of the rigid class distinctions below stairs in a nobleman's household where the chef has his meals in his own private apartment, the kitchen maids in their quarters, the chauffeurs, footman, under butler, and pantry boys in the servants' hall.
THE LEADERS OF THE GRAND MARCH
But most striking, it has always seemed to me, is the settled order in which trees march into the land. Why shouldn't the oaks come before the maples? Or the maples before the beeches? Or the beeches before the pines? Why is it that, with the exception of a straggler here and there, the first trees to climb the stony mountainsides are the pines? Then close behind come such trees as the poplars, and along the streams below, the willows. Still farther down the valley are the beeches; farther still the maples, and last of all the oaks.
So it is they advance in a certain regular way, each in its own place in the ranks. At first it seems as strange as the coming of Birnam wood to Dunsinane that gave poor Macbeth such a turn that time. But, after all, the explanation is quite simple and no doubt you have guessed it already.
The reason such trees as the pines, poplars, and willows come first is that the seeds are so light they are easily carried by the winds and so reach new soil ahead of other trees with winged seeds like the beeches and the maples; for, although these seeds also travel on the wind, they are much larger than the winged seeds of the pine and they travel much more slowly and for shorter distances.
Moreover, at the end of their first journey, having once fallen to the ground, they are apt to stay. Then there is no further advance, so far as these particular seeds are concerned, until trees have sprung from them and they, in turn, bear seeds. In the case of very light seeds, like those of the pines, the wind not only carries them far beyond the comparatively slow and heavy march of the beech and the maple, but if they fall on rock with little or no soil the next wind picks them up and carries them farther, so that they may strike some other spot where there is soil and perhaps a little network of grass and weeds to secure them until they can take root and so hold their own. It is not only a great advantage to the pine seeds to be so small, so far as getting ahead of other trees is concerned, but it is an advantage in another way. Because they are so small they require comparatively little soil to start with, are more easily covered up, and so they soon begin to sprout. The very winds that carry them up among the mountain rocks are quite likely to cover them with enough dust to start on, and I myself have helped raise many a giant of the mountain forests in this way. It is really wonderful how little soil a pine-tree can get along with; if, say, its fortunes are cast on some mass of mountain rock. Somehow it manages to get a living among the cracks and at the same time to hold its own in the bitter struggle with the winds.
"The pine trees," says Muir, "march up the sun-warmed moraines in long hopeful files, taking the ground and establishing themselves as soon as it is ready for them."