450 THE POPULAR SCIENCE MONTHLY 



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 Pellcea, Notholcena 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 canon in a few steps we find ourselves again among the ocotil- 

 los 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. Matur- 

 ing 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 grow- 

 ing 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 Mex- 



