* SUDD "-FORMATION OF THE UPPER NILE. 55 
seen the whole channel of the Bahr el Arab choked with rhizomes 
of the Panicum, while the rhizomes of the Phragmites, floating in 
the Nile channel, are often 15 to 20 yards long. 
The rafts formed by these plants were further strengthened by 
twiners, which bound them and the shoots they gave out together. 
Among these Ipomea reptans, which, as I have mentioned before, 
has also tubular rhizomes and is thus able to reach the most 
distant shoots, is the most important; but the other twiners, 
as well as other plants, were soon able to germinate in the soil 
deposited on the surface of the raft. Their roots and the soil 
which they gathered, as well as floating waterweeds and vegetable 
rubbish brought by the wind or currents, filled the interstices of 
the raft and further eonsolidated it. These masses of vegetation 
dropped their roots into the soft ooze at the bottom of the lake; 
but a rise in the lake or a storm broke away portions of the rafts, 
whieh drifted away until they anchored themselves in other 
shallow places, where they again spread. These movements in 
the masses of vegetation still take place, and have been fully 
described by Sir William Garstin. 
Although Ambatch is, at certain places in the swamps, e. g. the 
southern part of the Sudd and in the Bahr el Ghazal swamps, 
very abundant, Sir William Garstin is of opinion that it has no 
claim to rank in importance, as a Sudd-forming plant, with the 
species mentioned above. I quite agree with him and believe 
that it only appears, with other plants, after the rafts of floating 
vegetation have formed themselves, and that. at the best, it only 
acts as anchors wbich help in fixing the rafts in one place and 
thus form firm land. 
That the soil is slowly forming and filling the bed of these 
swamps, there is little doubt. The Nile in flood at Gondokoro 
is well nigh as turbid as the Blue Nile when the latter is in flood ; 
but a large proportion of this silt gets deposited on the roots of 
the plants in the swamps, and in still waters the mud sinks to 
the bottom. Successive layers of decomposing vegetation should 
add materially in forming soil, and seeds of large riverside treos, 
washed from above, may germinate and help in the work of 
consolidation ; but, unfortunately, fierce fires sweep over the 
whole country and the ashes of the plants are carried awav 
by the wind and a large portion deposited in the channels and 
carried away, while what seedlings of trees do germinate are: 
with few exceptions, killed outright by the fires. 
