921 MR. H. H. W. PEARSON ON THE 
and to explain these several theories have been advanced (33). 
Of the factors which have been important in the selection of 
xerophytes or in the adoption of xeropbytie charaeters on the 
humus-patanas, the following are probable :— 
(1) The bad ventilation and comparative low temperature (36) 
of the soil, due to the presence of hydrostatie water, and 
the eonsequent lowering of the respiratory activity of the 
roots. 
(2) The high power of capillary absorption possessed by a 
peaty soil (33), by reason of which the absorption of water 
by the roots is less than in any other soil containing an 
equal proportion of water. 
(3) The presence of the humus acids of the soil, which still 
further impair the activity of the roots (31). 
These three factors combine in lowering the functional 
activities of the roots ; and since the funetions of the aerial parts 
must be in correspondence with those of the roots, the acquisition 
of xerophytie characters has been necessary. 
The flora of the dry patanas, i. e., speaking generally of the 
patanas below 4500 feet, compares in many respects with that 
of the South American savannahs (37). 
There is, for example, an absence of plants with bulbs or 
tubers, and in the following list of patana-plants true succulents 
are rare. This is probably due to the fact that the dry season, 
though long, is not excessively severe, and the wet season, in 
which the Uva vegetation wakes up to renewed activity, is not 
extremely short; and therefore such an effectual protection 
against evaporation and such a large storage of water as a bulb 
or tuber affords to enable the plant to endure excessive drought, 
and afterwards to pass rapidly through its vegetative aud 
reproductive periods during a very short wet season, are 
unnecessary. Another point of correspondence between the 
savannahs and the patanas is seen in the large proportion of 
perennials as compared with the annual species : this is true for 
the dry patanas, and equally so for those situated above 4500 fet. 
The perennials constitute 86 ?/, of the flora above 4500 feet, 
aud below 4500 feet 87:3 °/,. In general a dry climate is 
favourable to the developinent of annual species (38), since the 
seed or the fruit is obviously the best form in which a plant can 
tide over a dry season. On the patanas, however, as on the 
