Wherever soil is coarse and bouldery the weeds also are of a sturdy breed. In his long, delightful days among the mountains Muir[4] tells us what a brave show the thistles made in this new world of soil; how royal they looked in their purple bloom, standing up head and shoulders above the other plants, like Saul among the people. [4] Muir. "The Mountains of California." [Illustration: WHAT THE DESERT PIONEERS DO FOR FUTURE GENERATIONS Only the sturdiest kinds of shrubs and weeds, such as you see in the desert, can earn their keep in sandy soil, always thirsty, like that on the right. But the desert vegetation, dying and decaying--it is then called "humus"--not only knits the soil together but absorbs moisture and ammonia from the air and so helps grow good crops.] HOW PLANT PEOPLE PAY THEIR TAXES 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. [Illustration: 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.