250 



NATURE 



[April 28, 1910 



AMERICAN DESERT VEGETATION.' 



'X'HE popular impression of a desert as an endless 

 J- plain of tawny sand, rainless, and utterly devoid 

 of vegetation, or perhaps showing a distant oasis, bears 

 but slight resemblance to the desert overlooked by the 

 botanical laboratory near Tucson. Here considerable 

 variety of vegetation prevails; in the streams and 

 river aquatic plants flourish ; along the river banks 

 rise poplars and willows ; on the alluvium of the 

 " flood-plain " is mesquite-forest, in which acacias and 

 another leguminous species, Prosopis velutina, live 

 side by side with elder-trees and ash-trees ; approach- 

 ing the hills other types of vegetation appear in the 

 dried water-courses, and on the gravelly and sandy 

 slopes, in both of which sites grows the notorious 

 creosote-bush (Larrea) ; while on the hills are found 

 yet other plant-communities, including giant cacti 

 and Fouquieria. In the winter and summer seasons 

 of rainfall — scanty though this be — the scene changes 

 like magic, for thousands of short-lived annual 



case near rivers, atmospheric factors militate against 

 luxuriance of growth or multiplicity of species, iln 

 less moist soil desert-plants evade or withstand the 

 danger of desiccation by their possession of peculiar 

 characters that may be physiological or morphological 

 and anatomical. 



As regards physiological peculiarities, the desert 

 plants that are capable of reviving after thorough 

 desiccation are few in number and are limited to 

 lowly organised types, such as lichens (yet these by 

 no means lack protective arrangements, as is indi- 

 cated in Dr. Fink's article on lichens in the volume 

 under review). On the other hand, many flowering 

 plants exhibit in their life-history a rhythm that 

 enables them to thrive in the desert without the aid ol 

 any adaptive structural features. For instance, in 

 deserts there are many ephemerals that spring up in 

 the rainy season, and within a few weeks produce 

 leaves, flower and fruit, and die. They evade the 

 true desert conditions, and survijve in virtue of their 

 rapid completion of the life-cycle at a definite season. 



Fig. I. — Ri^ht siJe of gulch n^ar Laboratory, with generally sou.h exp jsu'e. 



(ephemeral) herbs spring up and clothe the ground 

 with fresh verdure that contrasts with the ashen or 

 bluish-green tints of the bushes or bizarre succulents. 



Variety of water-supply, of slope, and of soil (clay, 

 gravel, sand, alluvium, hard pan, saline spots) evoke 

 corresponding variety in the vegetation of this patch 

 of desert, and render the site eminently suitable for a 

 botanical laboratory and for the solution of ecological 

 and physiological problems by observations and ex- 

 periments on desert platits in their natural surround- 

 ings. 



Desert plants are exposed to the danger of death 

 from desiccation by reason, first, of the various in- 

 tense climatic factors tending to cause excessive 

 evaporation, and, secondly, by the scantiness of the 

 water available for absorption by the roots. Hence 

 even where water is abundant in the soil, as is the 



1 " Distribution and Movements of Desert Plants." By V. M. Spalding. 

 Pp. V4-144. (Washington : Carnegie I-n-ti.utioir, 1909.) 



NO. 21 13, VOL. 83] 



In the Effvpto-Arabian desert there is but one annual 

 rainy season, namely, in winter, and consequently 

 only one annual crop of winter-ephemerals. Near 

 Tucson, however, there are two rainy seasons— in 

 winter and summer respectively — and corresponding 

 crops of winter-ephemerals and summer-ephemerals. 

 These two plant-communities consist of entirely 

 different sets of plants, the seeds of which (according to 

 the information in this volume) will not germinate at 

 the particular rainy season during which they are wont 

 to be inactive. 



The structural characters enabling desert plants to 

 exist have been dealt with by Volkens and other 

 investigators, and additional details have more re- 

 cently been supplied by workers at the Tucson labora- 

 tory. But the main value and novelty of the work 

 conducted in connection with this laboratory lies in 

 the investigation of the behaviour and physiological 

 activity of representative species, also in the thorough 



