FLORA OF THE INDIAN DESERT. 277 



The few annuals observed have generally a comparatively long taproot, the 

 exceptions from this rule being best regarded as accidental visitors to the region 

 ■{such as Spennacoce striata, Aspliodelus tenuifolius). — The rain perennials are 

 also visible above ground only during the rainy season, but have a perennial 

 underground stem. Here belong the bulbous Monocotyledons, of which Dip- 

 cadi erythroium is a representative from our region, also various Cyperacece. 



By far the largest number of the indigenous plants are capable of absorbmg 

 water from deep below the surface of the ground by means of a well developed 

 root system, the main part of which generally consists of a slender, woody tap-root 

 of extraordinary length. This adaptation in some cases enables a plant to dis- 

 jjense with all xerophilous characteristics. A noteworthy example is Citndlus colo- 

 cynthis, one of the Cncurbitacece, which remains green throughout the year, in 

 spite of its long, trailing branches, which often reach 50 ft. in length, and bear 

 a fair number of large leaves. A fruiting sjiecimen of the plant is shown on 

 Plate VII-A. 



Generally, however, various other xerophilous adaptations are resorted to 

 such as reduced leaves, thick tomentum, succulence, coatings of wax, thick 

 cuticle, protected stomata, etc., all having for their object a reduction of transjii- 

 ration. The plants belonging here are chiefly more or less woody perennials. 

 A few annuals occur, however, svich as the rare Monsonia heliotropioides. 



8. Formations. 



It was originally our intention to adopt the nomenclature used by F. E. 

 Clements in his work " Plant Succession " (Washington 1916) for our descrip- 

 tion of the vegetation of the Indian Desert. For many reasons, into the details 

 of which we cannot enter here, this plan has been abandoned. Accordingly 

 the term " consocies " used on some of the earlier plates (PI. VII-B., X-A.) 

 should be replaced by the term " family." Similarly the words " in the con- 

 socies " under PI. X-B. should go out. 



The uniformity of the climate of our region causes a corresponding unifor- 

 mity of the vegetation. The formations may therefore be taken as exclusively 

 edaphic, and it is convenient to adopt Schimper's definition : " The communi- 

 ties of plants as determined by the quahties of the soil are termed formations." 



Accordingly, we distinguish the follomng five formations : Aquatic, Sand, 

 Gravel, Rock, Ruderal. 



For the sub-divisions of the above formations we use the term association, 

 folloA^ing Warming's definition : " An association is a community of definite 

 floristic composition wthin a formation." 



We shall further use the term family, introduced by Clements, but in the 

 following generahzed sense : A family is a community of individuals belonging 

 to a smgle species, and occupying a definite area of whatever shape or size, the 

 boundaries of which are determmed by the numerical distribution of the mdivi- 

 duals, no account bemg taken of the eventual occurrence of other species within 

 the area. 



Thus PI. X.-A. shows a smgle family of Edij^ta erecta with abrupt boundaries, 

 the change in numerical distribution within the occupied area being continuous. 

 In PI. X.-B. we have several isolated families of the same plant. In the case 

 discussed no other plant occupies the same locahty, and we may therefore des- 

 cribe the local vegetation as a pure association of Edij^ta ereda^ consistmg of 

 several families. A pure association may coincide with a family, as in PI. XXI V- 

 A., XXV-B. {Cyperus arenarius), or form part of a family, as in PI. XX\ -A. 

 {Calotropis procera) or VII-B. {Indigofera argentea). The area occupied by a 

 family of Aristida hirtigluma sho\\Ti in PI. XIX-B. contains also other species 

 -and hence the plant does not form a, pure association ; this is of course the rule. 



