306 THE JOURNAL OF INDIAN BOTANY. 



2. The biotic factors. 



The term " biofcic factors " includes all influences traceable 

 to living organisms, but only animals and man are considered in 

 this paper. Undoubtedly man and his domesticated animals are 

 the most important of the biotic factors. For more than 20 

 centuries the Gangetic Plain has been populated with an agricultural 

 people. It is difficult and perhaps impossible to form any adequate 

 conception of the intensity of the human factor in times past, 

 but since 1850 the population of the 2811 square miles of Allahabad 

 District has fluctuated from 480 to the high level of 543 to the 

 square mile at the 1891 census (8). At present it is about 530 per 

 square mile. To the human population must be added, according to 

 census of 1909, 331 cattle and buffaloes, 123 sheep and goats, and 

 8 horses and donkeys, or a total population of domestic grazing 

 animals of about 463 per square mile. Therefore the area has to 

 support a total population of about 1,000 per square mile of animals 

 that gain most or all of their food directly from the vegetation. 



Man influences the vegetation in a number of ways, mainly by 

 cultivation, by grazing his animals, and by cutting for food and fuel. 



Cultivation. The returns of 1907-08 (8) show that about 58 per 

 cent, of the District was under cultivation. This figure probable fairly 

 indicates the extent of cultivated land from year to year in the area 

 immediately surrounding the city of Allahabad. Cultivation causes 

 retrogression of the vegetation, and the more thorough it is, the great- 

 er the effect. Wild plants, both annuals and perennials, are rooted 

 up and killed, and their place is taken by annual ruderals, usually 

 native, but in many cases introduced. At the same time cultivation 

 tends to make the area more xerophytic by removing the permanent 

 plant covering, and substituting a cover of perhaps more mesophytic 

 but short-lived annuals. "When the crop cover is harvested the soil is 

 left practically bare, and dries out speedily. 



Grazing animals. All of the uncultivated land in the area is 

 closely grazed throughout the year. During the rainy season, the 

 vegetation is able to keep pace with grazing, even though it is con- 

 stantly kept eaten off close to the ground. As the cold season 

 progresses and the growth of herbaceous vegetation is checked, the 

 effect of overgrazing become more and more evident. Finally, during 

 the dry season all grasses and associated plants are eaten down to 

 the soil surface (Fig. 11). Annuals die under the combined hardships 

 of the grazing and lack of protection. Only perennial grasses and a 

 few other xerophytic herbs with strong perennating organs are able 

 to survive. Where overgrazing progresses still further, even the peren- 



