390 XXIII. LEGUMINOSAE 



extent greater in lower than in upper Sind, the babul being favoured bj' these 

 moister conditions. The occurrence of Prosopis spicigera marks one of the 

 successive stages between that in which new alluvial ground is formed in the 

 river-bed and the time when, owing to the receding of the river, this land is left 

 high and dry above the reach of even abnormal river floods. The flood season 

 of the Indus commences about the beginning of May, with the melting of the 

 Himalayan snows, and the river continues to rise, with occasional interrup- 

 tions, until July or sometimes later ; thereafter it subsides gradually and 

 reaches its winter level as a rule about the end of October. In the course of 

 the annual floods much land is lost by erosion, while corresponding new land 

 is won by accretion. The newest so-called kacha, alluvium thrown up by 

 deposits of silt becomes quickly covered with Tamarix dioica and T. Troupii, 

 and a growth of Jcanh grass {Saccharum spontaneum) : as the ground becomes 

 more elevated Populus euphratica and Acacia arahica make their appearance, 

 while later, when the land becomes elevated above the reach of all but abnormal 

 floods, Prosopis spicigera becomes the prevailing species, having already 

 gained a footing with the aid of the river floods and having survived after the 

 preceding species have gradually disappeared. This, however, is not the 

 final stage, for although its long taproot and its power of reproduction by 

 root-suckers enable it to outlive its predecessors it is not strictly a desert 

 species, and though it lingers for a long time on the older higher and drier 

 alluvium it eventually gives place in the driest tracts to more typical desert 

 species such as Capparis aphylla, Salvadora oleoides, and S. persicu. 



While often gregarious, particularly in upper Sind, Prosopis is frequently 

 associated by single trees or small groups with the various species which 

 precede it on the one hand and succeed it on the other. Thus, before it meets 

 the desert species it may be found with Tarnarix dioica and Troupii (some- 

 times in the form of moribund remnants). Acacia arabica, Populus euphratica, 

 and Tamarix articulata, the largest species of Tamarix, which is characteristic 

 of drier ground than the other two : again, on the older and drier alluvium 

 Prosopis is commonly associated with the desert species already mentioned, 

 with or without surviving individuals of the other species. 



The soil on these alluvial tracts consists of varying mixtures of sand and 

 clay. A special feature, indicative of deterioration of the soil through lack of 

 surface drainage, is the presence of tracts of varying extent in which the soil 

 is charged with an excess of sodium salts, these salts appearing as a white 

 efflorescence on the surface of the ground. Where the soil becomes very 

 saline Prosopis, in common with most other species, quickly dies out ; on 

 pure sand, also, it does not survive long. 



In the Punjab Prosopis spicigera occurs throughout the alluvial plains 

 from the Salt Range to the Sutlej river, but does not extend into the hills. 

 By far the largest tracts are situated in the arid regions in the south-west of 

 the province in the Mult an and Montgomery districts. According to Mr. 

 B. O. Coventry,! the area of these forests under the Forest Department in 

 1915 was about 3,500 square miles, of which about 2,700 square miles were 

 in Multan, and about 700 square miles in Montgomery, the remainder being 

 in the Lahore, Gujranwala, and Shahpur districts. There is little doubt that 



1 Ind. Forester, xli (1915), p. 307. 



