54 MR. A. F. BROUN ON THE 
bank, but above Kenissa, the site of the old Austrian Mission of 
Ste. Croix mentioned by Sir S. Baker, the course gradually 
approaches the right bank, which only becomes visible far in the 
distance some way above Shamba. At Bor it actually flows under 
the high right bank. Above this point the main channel goes 
from one bank to the other and back again, and, although the 
marshes between the high banks on either side are still of con- 
siderable width (at least as far as Mongalla, our southernmost 
post, 474 miles above Lake No), the boundaries of the Nile Valley 
above Bor are well defined. 
As regards the vegetation of this part, it is striking that some 
way above Shamba the banks begin to be better defined and that 
Papyrus gives way gradually to Om-Suf, the former being, in the 
southern swamps, only represented by clumps or belts denoting 
depressions in the surface of the ground. The grass-covered 
surface is dotted here and there with trees, either solitary or in 
clumps, chiefly Crateva religiosa, Forst., Kigelia æthiopica, 
Deene., and Ficus Sycomorus, Linn., the low banks being often 
covered with fairly dense fringes of Sesbania egyptiaca, Pers. 
It appears to me most likely that, at some time more or less 
distant, the Nile debouched near Bor into a large lake, the 
northern limit of which was marked by the rise of the ground 
towards the Nuba hills in Kordofan, still marked by the Bahr el 
Ghazal and the White Nile flowing from west to east as far as 
the Sobat mouth. It is possible, and even probable, that this 
lake did not, at first, occupy so wide an area as is now covered 
by the swamps, but that it gradually became filled with silt, the 
cone of dejection spreading northwards from Bor. This would 
account for the present disappearance of Papyrus im the southern 
portion of the swamps. 
It is also easy to conceive how, as the waterin the lake became 
shallower, its surface was covered with vegetation. Among the 
plants growing on the shores of the lake were some furnished 
with more or less tubular rhizomes which they sent out over the 
surface of the water, and, being numerous, crossed and recrossed 
each other and interlaced, thus forming rafts, sometimes of con- 
siderable thickness and buoyancy. The most important of these 
plants are now the chief Sudd plants: they are Cyperus Papyrus, 
Panicum pyramidale, Phragmites communis, and Typha australis. 
Of these the Panicum and the Phragmites have not only the 
lightest and most tubular, but also the longest rhizomes. I have 
