308 THE JOURNAL OF INDIAN BOTANY. 



white ants eat the remains. The great problem in this connection is 

 whether and to what extent they prevent the accumulation of this 

 dead vegetation as organic matter in the soil. It may be that the 

 intense heat and dryness of the hot season would oxidize organic 

 matter even if it were introduced into the soil, though the. experience 

 from trenching indicates that this is not necessarily the case. It is 

 probable that the organic matter eaten by white ants is oxidized 

 within their bodies. This prevents the usual course of oxidation by 

 the action of bacteria in the soil with the resulting increase of 

 nitrates and solution of mineral constituents, and the soil is depri- 

 ved of just so much fertility. In so far as they prevent the accumu- 

 lation of organic matter in the soil, and thus impair its fertility and 

 water-holding capacity, white ants are detrimental to the vegetation. 

 Almost literally every square inch of ground is either cultivated 

 or grazed. Excessive grazing is by far the most important of the 

 local biotic factors. It becomes increasingly evident that there can 

 be no adequate interpretation of the vegetation of such an area as 

 the Upper Gangetic Plain without due consideration of the tremend- 

 ous pressure of the human factors. Neither climatic nor biotic 

 factors alone, but all acting together, influence and determine the 

 vegetation. 



Ecological features of the vegetation. 



Types of succession. It is well established that under natural 

 conditions the vegetation of new local areas, such as ponds, 

 stream margins, cliffs, bare rocks, cultivated tracts, and the like, 

 passes through a succession of associations, finally culminating 

 in a permanent climatic climax. Cowles (3) distinguishes two 

 types of succession in any given region : — (l) Climatic, depending 

 on slow changes in geologic climate, and which is at all times in the 

 stage of climatic climax ; and (2) Topographic, occurring as smaller 

 cycles of development within the immensely large climatic succession, 

 and dependent on local physiographic variations. The climatic 

 succession progresses so slowly that it cannot be made the subject of 

 exact study. Topographic succession progresses much more rapidly, 

 but still is a slow process. It usually is assumed that the associations 

 observed in passing from a new area to the climax fairly represents 

 the succession as it occurs at any given spot through a long period of 

 years. Each topographic succession area has as its goal the climatic 

 climax vegetation, and is terminated when this stage is attained. 



Seasonal succession. A third typeof succession is a prominent 

 feature in a strongly periodic climate. It is illustrated by the 



