Page:The Journal of Indian Botany.djvu/354

308 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 deprived of just so much fertility. In so far as they prevent the accumulation 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 tremendous pressure of the human factors. Neither climatic nor biotic factors alone, but all acting together, influence and determine 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