1923. | Ecology of Riverine Tract of Burma. 95 
is well exemplified in Burma. In the region studied, which lies 
entirely outside the wetter areas in which the Rain Forests 
occur, there is every gradation from Monsoon Forest (espe- 
cially the Teak Forests and Indaing) through Savannah Forest 
to Thorn Forest, Thorn Scrub and Semi-Desert. Everywhere 
the principal undergrowth in grass, proving that the climate 
is not hostile to its growth, though slightly more favourable 
to tree-growth. 
When working in any one area one is tempted to say that 
the soil, and almost the soil alone, is the determining factor 
in the distribution of vegetation. There are many places in 
which may be found within a stone’s throw of one another 
typical Monsoon Forest of Eng and Ingyin ; Savannah Forest 
of Tectona hamiltonii and Terminalia oliveri ; thorn forest of 
Acacia catechu and thorn-scrub of Acacia catechu or Zizyphus 
jujuba, so powerful is the edaphic factor. It is when one 
Studies the region as a whole that one realizes that the cli- 
mate, especially the rainfall is really the main determining 
actor. 
of the soils—that is really on the basis afforded by a study 
of the geology, the following points may be noted. 
In the south—near Prome—the Irrawadian sands and 
gravels are covered by Teak Forest and Pentacme—Dipterocar- 
pus Forest (Indaing). Both require a well drained soil, but 
the Indaing flourishes on extremely light soils—really pure 
sands. To be seen to perfection Teak requires a rainfall 
higher than that of Prome (47°25 inches) and the Teak forests 
near Thayetmyo and Allanmyo are “ stunted,”’ the trees having 
a girth of less than 5 feet at a height of 5 feet from the 
ground. Teak is scarcely seen north of Thayetmyo (37°34 
inches) but the Indaing stretches somewhat farther north and 
then passes into a mixed forest described epee < the 
e Ir 
Tf one considers the vegetation firstly from the standpoint 
tation of the Irrawadian near Yenangyaung (25°52 inches). 
t may be noted in passing that bands of clay in the Irra- 
wadian may cause local changes whilst level plateau surfaces 
are often covered with a hard ferruginous layer which only 
supports a stunted Thorn—Forest or Thorn scrub. 
whole, decid- 
to Thayetmyo and then to the north-west there are large 
areas of Tectona hamiltonii—Terminalia oliveri Forest. Wher- 
ever drainage is bad—especially on level plateau surfaces—this 
gives place to the Acacia catechu Thorn Forest. As soon as 
