BOTANY OF THE CEYLON PATANAS. 321 
prevailing opinion among those who are best able to judge is 
that, where not interfered with by fire, the forests tend to 
reinstate themselves. This process is, however, even under the 
most favourable cireumstances, very slow, being hindered by 
several of the well-known properties of humus. 
Humus has a greater capacity for water-absorption than any 
other soil (33), a fact which militates against the spread of forest- 
trees. By it the normal distribution of water in the soil is 
disturbed, the upper layers becoming wet at the expense of the 
lower. Asa consequence the germination of the seeds of forest- 
trees is hindered by the excess of water near the surface (32), and 
at the same time, at a greater depth, the soil is rendered too 
dry for the nourishment of deep-rooting trees. The normal 
respiration of the roots also is impaired by the presence of 
hydrostatic water, and consequent poverty of the soil in free 
oxygen (32). A further consequence of the excess of water 
in the humus is that the soil-temperature is below that which is 
normal for the latitude and elevation—a fact which must have a 
considerable effect upon the germination of seeds (32). There 
are no observations to show what are normal soil-temperatures 
for given soils in such a locality as this ; but it may be interesting 
to compare the following rough determinations made upon the 
humus-patanas ata depth of 4 inches, with those made on the dry 
Uva patanas at lower elevations (v. p. 313) :— 
Hak gala (5600 ft.). 66? F., Aug. 19, 9.30 A.M. 
» » 67° F., Oct. 14, 12.30 P.M. 
» » 66? F., Nov. 3, maximum. 
» » 61? F., Oct. 26-27, minimum. 
Sita Eliya (5800 ft.). 63?:5 F., Oct. 20, 9 A.M. 
Horton Plains (7000 ft.). 59?2 F., Aug. 10, noon. 
Of considerable importanee in hindering the advance of 
forest-growth over the humus-patanas are the acid properties 
of the soil. The humus-acids produced in the superficial layers 
sink to lower depths and render the lower soil acid—a condition 
which is known to be very disadvantageous to the growth of 
most European woody species (31), but favourable to the 
Rhododendron and a few others. We have further, in the 
acidity of the humus, and the abundance of ferrous oxide which 
is the predominant colouring-material of the soil underlying it, 
conditions which are in every way favourable for the formation 
