544 THE POPULAR SCIENCE MONTHLY ' 



tion has long been current, and I must plead guilty to some participa- 

 tion in statements tending to perpetuate the mistake. It may be easily 

 found, however, that even the maximum pressures noted above would not 

 retard transpiration as much as ten per cent, from that which would 

 take place with a sap of distilled water. One of my reviewers has re- 

 cently made a variation of this mistake in suggesting that acidity would 

 have a retarding effect on water loss. No foundation exists for such a 

 supposition. 



Almost any ordinary branching plant with broad leaves will, if 

 forced to carry out its development under arid conditions, show some of 

 the features of the type of desert plants described, and it is customary to 

 assume that the causal conditions responsible for such forms are the 

 desert factors : that we have here a direct adaptation or environic re- 

 sponse which has become heritable in the strictest and fullest sense. 

 This is a matter that deserves the fullest consideration. Meanwhile it 

 will be perfectly safe to assume that such spinose forms represent the 

 simplest or most elementary specializations of desert plants, and species 

 with the most diverse morphological constitution may show alterations 

 of this character. The sclerophylls of the American desert include 

 species of Prosopis, Acacia, Calliandra, Parkinsonia, Cercidium, Olneya 

 of the leguminous plants, Covillea and Zizyphus of the Zygophyllaceas, 

 Fouqueria, Lycium, Koeliberlinia, Condalia, Manzanita, Franseria, 

 Jatropha, Sapindus, Vauquelinia, Quercus, Aster and others. 



Southwestern America has been arid for an extremely long period, 

 not uniformly so, however. The researches of Professor Ellsworth 

 Huntington, in which evidence has been obtained from ruins of struct- 

 ures built by man, of geological terraces, lake beds, strands and drainage 

 lines in Central Asia, Palestine, and America, and also by the examina- 

 tion of the structure of the big trees of California, seem to justify the 

 conclusion that variations in climate with regard to temperature and 

 moisture have taken place within the last two thousand years that would 

 be of profound biological importance. It seems fair to assume that 

 similar oscillations, each movement of which might extend over a few 

 hundred years, have taken place previously. 



It is under these conditions therefore that we are to think of the 

 evolution of the desert vegetation of the southwest, and present knowl- 

 edge compels us to believe that much of it originated somewhere within 

 the limits of the region which is arid at the present time. Perhaps 

 the most important constitutents of this indigenous specialized flora 

 are the cacti, which must have originated somewhere in the Mexican 

 highlands in the Tertiary or later. This group is known to contain 

 over a thousand species, and now extends through South America, its 

 distribution offering some most highly localized occurrences of species. 

 So rapid has been its evolution, and so wide the amplitude of its depart- 



