Page:Popular Science Monthly Volume 75.djvu/454

450 mountain top the conditions favorable to its growth and it attaches indiscriminately to any woody plant that furnishes a convenient hold.

But of the many things that we brush by in this ramble we have not the time to tell, but in this narrow space of moisture between zones of perennial drought, occur ferns of the genera Pellæa, Notholæna and Cheilanthes. Under the neighboring rocks their prothallia are growing, and young sporophytes of all ages are coming on. Climbing out of the bed of the cañon in a few steps we find ourselves again among the ocotillos and the agaves and the cacti.

Among the shrubby plants none are so important as the guayule, the native name for Parthenium argentatum. It is one of the most abundant of all the desert plants, especially on the limestone slopes, and its grayish color gives a distinct character to the landscape where it abounds. A small shrub or dwarf tree, it seldom exceeds four feet in height or a stem diameter of four inches. Its leaves are covered with silvery hairs and its flowers are in inconspicuous heads of composite structure not over one fourth inch across. Its light seeds—one hundred would not fill half an ordinary thimble—are supplied with a papery bract by the aid of which they are driven easily by the wind. Maturing in late summer and autumn, the seeds are dropped to the ground beneath the parent plant, or by some strong gust of wind are borne to a distance, where some find lodgment in a sheltered spot—a crevice of the rock or the cover of some friendly shrub. Here, when the rain comes, it is kept moist for time enough to send down a long, slender, thread-like root before drought again overtakes it. After the fitful showers of summer have passed a long dry season awaits the young plant, so it behooves it to make as much root as possible while the growing conditions are favorable. These slender roots will make a growth of six inches in about a week, before the first true leaf has appeared lifted on the short stem half an inch high, and in six weeks the tap root has been observed fifteen inches long. Thus the plant insures itself against the dry season, and by hardening its stem and leaves, makes still further provision against the vicissitudes that await it.

And this example serves, doubtless, for many other desert plants. We find that the seedlings spring up in abundance under the shelter of bushes and cacti and other perennials. In fact, elsewhere there is almost no chance for the survival of a tender seedling, since the hot sun dissipates so quickly the moisture of even a heavy rain that the surface of the soil is again dry in less than a day. The chances of survival of a seedling are exceedingly remote, and considering the great number of seeds produced and distributed, probably only a very small fraction of one per cent, even germinate.

But interest in this guayule which covers the desert slopes is not alone in relation to its environment, but in the hule or gum which it produces, forming no small part of the rubber production of