OP CEYLON AND INDIA. 
277 
those plants which become exposed to the air soon die, and 
so we see a sort of tidemark traced upon the rocks by their 
covering of Podostemaceæ. This mark indicates the average 
level to which the water falls during their growing season, 
above which it may temporarily rise during heavy rain, or 
below which it may sink during exceptional dryness of the 
weather. 
Plate IV. shows a view of a portion of the rapids at 
Hakinda, photographed in the dry season, with fruiting 
Podostemaceæ on the rocks. These plants do not show very 
distinctly in a photograph, owing to their dull gray colour, 
which is not very distinct from that of the rock itself, but it 
is not difficult to make out the band of exposed dead plants 
forming a kind of tidemark on the rocks for some few feet 
above the present water-level. At the places of most violent 
rush of water the thalli are chiefly the flat closely attached 
Lawias and Hydrobryums, while in the less violent parts the 
Dicræas occur, and in the eddies Podostemon and Farmeria 
are abundant. 
The Rainfall and its Distribution . — Perhaps the most 
important factor in the life-history and ecology is the rainfall 
and its distribution throughout the year, as this factor 
controls the level of the Avater, and hence the local distribu- 
tion and the period of flowering of the plants. The following 
table, taken from the meteorological report of the Surveyor- 
General for 1898, gives the average rainfall for twenty-six 
years at Kotmale (Gingran-oya),in the centre of the mountain 
basin of the Mahaweli-ganga ; the distribution of this fall 
is a fair index of the general state of the water-level of the 
river at Peradeniya at the various times of the year : — 
Month. 
Inches. 
Days. 
Month. 
Inches. 
Days. 
January ... 
2-93 
... 7 
July 
21*41 
... 23 
February,.. 
2-19 
... 5 
August 
16*59 
... 22 
March 
4-92 
... 8 
September ... 
15*44 
... 20 
April 
9*34 
... 15 
October 
17*60 
... 23 
May 
12*11 
... 16 
November ... 
11*72 
... 16 
June 
24*99 
... 25 
December ... 
8*36 
... 13 
