186 TREUBIASV Or I ET 
(within the limits under observation: less than 3'/,°/99 —84.6°/o,) on the 
occurrence of Chaetomorpha. 
Still, in the Batavia empangs the large masses of Chaetomorpha and 
speaking more generally the whole submerged vegetation develop most luxu- 
riantly in spring at the end of the West-monsoon, i.e. ata period when the 
salinities of the fish-pond water (except perhaps in a very abnormal year) are 
always lower than 30 °/,, usually below 20 °/9, occasionally even below 5 °/o9. 
But it cannot, on the strength of this, be assumed with certainty that 
low salinity is actually the factor that favours the strong development of 
Chaetomarpha masses and of the submerged vegetation in general. For 
during the West-monsoon also the salinities are low or even very low, 
and yet the most luxuriant development of the masses of Chaetomorpha 
and of the submerged vegetation in general takes place, not during but 
after a pronounced West-monsoon. What may be of importance in this 
connection is the fact that during the West-monsoon the heavy falls of 
rain render conditions in the ponds rather unstable, in other words that 
during a pronounced West-monsoon the water in the fish-ponds is very 
little in repose. | 
There are however, a few more indications in connection with which 
it seems probable that a luxuriant development of the submerged vegetation 
is favoured by salinities of say below 30°/,, or little higher than 25 °/,- 
The ponds mentioned in Chapter Il to which water of low salinity can 
nearly always be admitted, such as those near Heemraad-Oost, are indeed 
on the whole distinguished by a more continuous luxuriant development of 
the submerged vegetation. 
Besides Chaetomorpha, as the chief component of the submerged ve- 
getation reaching up to just beneath the surface of the water in the Batavia 
empangs, one of the further important components to be mentioned in the 
first place is Ruppia rostellata KOCH '), a brackish and salt-water cos- 
mopolitan belonging to the Potamogetonaceae. 
Figure 4 (Plate XII) represents part of a Ruppia rostellata plant from 
one of the Batavia empangs. In Malay Ruppia is mostly called simply 
“rumput” (= grass) at Batavia. The branches and leaves of the Ruppia in 
the Batavia empangs mostly float in a level just beneath and against the 
surface of the water, in curved lines that remind the spectator somewhat 
of hair-vortices. 
Ruppia rostellata may be found nearly everywhere and at any time 
in the Batavia empangs. | usually found rather small patches of Ruppia of 
a metre or even less across, here and there, often amid the masses of 
Chaetomorpha, frequently also in places where the water was otherwise 
‘open. Very occasionally I observed how a somewhat larger part of 
a pond was occupied practically entirely by a Ruppia growth. Thus on 
') Determined by the Botanist for the Java-flora Mr. C. A. BACKER, 
