4 INDIAN FOREST MEMOIRS. 



tions and why it cannot exist under others, we shall be able to answer such questions as 

 whether or not the species can thrive in localities where it does not at present exist, and if 

 so what are the steps to be taken to make its successful introduction possible ; also why the 

 growth or reproduction of the species is unsatisfactory in a particular locality and what 

 steps can be taken to counteract its diseases. In other words we shall know what treatment 

 must be adopted in our Sal forests in order to secure their most satisfactory development 

 and reproduction and what steps must be taken to extend the area of such forests by intro- 

 ducing the species where it does not at present exist. 



While recognising therefore the vital importance of this study the magnitude and 

 difficulty of the work must be apparent to the most casual observer; involving as it does a 

 detailed knowledge of all the living and non-living factors of a plant's environment, of the 

 way in which, and of the extent to which, each such factor is capable of influencing favour- 

 ably or unfavourably the growth and development of that plant and of the extent to which 

 the favourable or unfavourable effect of these factors may be increased or diminished by 

 other factors. 



These points may be illustrated by a few simple cases : a plant A, for instance, is under 

 ordinary conditions of its habitat able to kill out a plant B by its competitive action and 

 this explains the absence of plant B in certain localities. In precisely similar habitats, 

 however, which are periodically fired, plant B is found to thrive and A to be absent, or 

 comparatively rare, and this is explained by the fact that plant A is more directly injured 

 by fire than B, or that fire has caused a change in soil and moisture supply more unfavour- 

 able to A than B. Firing, in consequence, is a simple treatment to adopt in order to favour 

 the development of B in such localities where A occurs. Again, species B may be unable 

 to exist on sandy soils owing to the small capacity of such soils for retaining moisture. 

 This capacity is greatly increased by an admixture of humus and the introduction of a 

 species A capable of thriving on dry sandy soil and of producing large quantities of humus 

 may be the best step to be taken with a view to the ultimate introduction of species B. 



In another case a species A may be unable to exist in clay soil owing to the supply of 

 oxygen being insufficient for the healthy action of its roots. Species B which is able to 

 thrive on clay soils and is less sensitive as regards the supply of oxygen may be capable of 

 improving such soils by the drying action of its roots and by the formation of humus, 1 

 and hence the introduction of species B may be the best procedure to adopt with a view to 

 the ultimate establishment of species A. Again the available nitrogen in a forest soil 

 depends largely on the rapid decomposition of the humus by bacteria and any factor which 

 retards the growth and development of these bacteria may result in so impoverishing the 

 upper layers of soil in respect of nitrogen that the seedlings of a particular species may be 

 unable to grow in them and the reproduction of that species thus becomes impossible. 



The production of good seed and vigorous seedlings also in many cases depends on the 

 existence of the birds and insects by which the cross-pollination of the flowers is effected 



1 " Humus acts as a weak cement and holds together the particles of soil, thus it serves both to bind a coarse-grained sandy soil, 

 and, by forming aggregates of the finest particles, to render the texture of a clay soil more open." The Soil by A. D. Hall, 1908, p. 47. 



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