BOTANY OF THE CEYLON PATANAS. 319 
The last two lines of the table give the monthly summaries 
of the daily temperature-observations made at Hakgala * during 
1897 (January to July) and 1896 (August to December). From 
these it will be seen that the air-temperature at Hakgala 15 
hardly more than that of an English summer. 
These conditions, viz., the extreme humidity of the air and the 
moderate and uniform temperature, are favourable to the accu- 
mulation of humus upon a soil which does not possess a high 
degree of porosity (26). It is, however, a well-known fact that 
humus does not accumulate in Indian and Ceylon forests except 
at high elevations; and in the forests of Ceylon, even at the 
highest elevations, the decomposition of plant-remains proceeds so 
rapidly that no considerable formation of humus can take place T. 
The experienee of foresters, however, shows that the removal of 
forest-growth, when the temperature is not too high, the rainfall 
not too low, nor the soil too porous, is favourable to the formation 
of humus-deposits, and, if the land has a suitable contour, to the 
production of swamps. The disappearance of the trees has a 
twofold effect in producing an increase in the humidity of the 
soil. It has been estimated that one-quarter of the amount of 
the rainfall never reaches the ground in a forest, owing to so 
much of the falling water being broken into spray as it falls 
upon the foliage and reabsorbed by the air (27); and, in addition 
to this, cleared ground loses much less water by evaporation 
than when its moisture is drawn upon by the roots of transpir- 
ing forest-trees (28), and this is especially true of a district 
where the air is always charged with moisture and where conse- 
quently the evaporation from the surface of the ground is slow. 
And further, the exposure to light occasioned by the removal of 
the forest-covering is in itself a cause of decrease in the rate of 
bacterial decomposition of organic remains (29). 
These factors have undoubtedly co-operated in the formation 
of the patanas above 5000 feet which are universally covered 
with humus-deposits except on some wind-swept patches at high 
elevations, and where they are interrupted by variously-sized 
* I desire to acknowledge the courtesy of Mr. W. Nock, the Superintendent 
of the Hakgala Meteorological Station, in supplying me with these and other 
statistics. 
T At Hakgala (5600 feet) it is impossible to obtain leaf-mould from the 
neighbouring forest for the Botanic Gardeus. 
