CHAP. Lxxii DESERT 277 



forms as Caralluma codonoides and Adenia globosa. Several kinds of 

 shrubs have tuberous stems, from which the branches spring. 



NORTH AMERICA 



Many districts in the south-west of North America are true deserts 

 or approximate to these (shrub-steppe).^ By the estabhshment of the 

 Carnegie Research Laboratory at Tucson in the desert of Arizona, and 

 the co-operation of botanists (Coville, MacDougal, W. A. Cannon) working 

 there, this desert will soon be the one of which most is known as regards 

 the form, biology, and physiology of plants. 



ADAPTATIONS IN DESERT 



Rapidity of development. In all these deserts one notes the same 

 astonishing rapidity of development of vegetation after the first showers 

 of rain and the commencement of spring. Fresh, green shoots suddenly 

 appear ; numerous and often beautiful flowers unfold on the parched 

 shrubs or start forth from the hitherto arid soil. 



Morphological features. In desert-plants one encounters a number of 

 structural features indicative of extreme xerophily, such as diminution 

 in size or arrest of leaves, production of aqueous tissue in stem or leaf, 

 sunken stomata, thick-walled epidermis and strong development of 

 cuticle, hairy coating, and other characters already described. Various 

 protective devices are often combined in the same plant .'^ 



Rolling plants in the desert. Not only in many deserts but also in 

 the closely related steppes, into which deserts often pass insensibly, one 

 hnds certain species of plants that break loose from the soil and are 

 buffeted hither and thither. These may be termed rolling-plants. Anastatica 

 hierochuntica, commonly known as the 'rose of Jericho', has long been 

 placed in this category, but incorrectly so according to Volkens. In any 

 ' case Odontospermum pygmaeum (Compositae) provides an example of the 

 phenomenon, and, according to Michon and Schweinfurth, is the true ' rose 

 of Jericho '. In South Africa there is the amarylHdaceous Brunsvigia whose 

 mfructescence, according to Bolus, is the sport of the wind, just as is the 

 infructescence of Spinifex on East Indian dunes.^ Finally, we may mention 

 the crustaceous lichen, Parmelia esculenta, belonging to lithophilous 

 vegetation in desert, which is torn from the rocks by storms, and is 

 transported by the wind in large quantities as ' manna ', until finally 

 , deposited at some distant spot : this phenomenon is common on all the 

 I deserts extending from Central Asia to Algeria. 



Hygrochasy is another character commonly exhibited by desert-plants. 



Stems, fruit-stalks, valves of fruits, and involucral bracts, are closely 



curled towards one another when dry, but open apart when moistened. 



By this means seeds are scattered only during the moist season. Such 



IS the case with Anastatica hierochuntica, Lepidium spinosum, Odonto- 



>permum pygmaeum, and Ammi Visnaga. On the contrary in a moister 



' limate many plants, including Daucus Carota, exhibit xcrochasy. 



' See Chapter LXXIII. 

 I ^ The anatomy and morphology of desert-plants are dealt with by Volkens, 

 X97 ; Massart, 1898; Jonsson, 1902 : Coville, 1893. The physiology of desert-plants 

 las scarcely been touched; but attention may be directed to papers by MacDougal, 

 \()0^, 1906, 1907 ; and Spalding, 1904. ' See p. 22/. 



