512 
Hope . — The ‘ Sadci ’ of the Upper Nile. 
‘Sadd’ in America. 
In conclusion, a comparison may be added with the American 
‘ Sadds,’ which have an entirely different composition. Mr. 
T. A. Sprague, of the Herbarium in the Royal Gardens, Kew, 
who has travelled across South America by way of the Amazon 
Valley, says that neither on the Amazon nor on the Orinoco 
do masses of floating plants occur in sufficient bulk to hinder 
navigation. This was not to be expected in such wide and 
deep rivers. But Mr. Sprague pointed out that something of 
the sort occurs in Guiana, and referred the writer to the 
very interesting book, ‘ In the Guiana Forest,’ by Mr. James 
Rodway, F.L.S. (T. Fisher Unwin, London, 1895), from which 
it appears that serious, though temporary, obstructions to the 
flow of the great rivers in that region (the Essequibo, &c.) are 
caused chiefly by two plants, Montrichardia arborescens and 
Panicutn elephantipes. At p. 107 of his book Mr. Rod way 
says : ‘ The tall trees cannot hold their own in the mud ; 
therefore they give place to a different type which has little 
or no trunk, and sits down as it were to anchor itself by means 
of special contrivances. Several species of Leguminosae, in- 
cluding Drepcinocarpiis Innatus , Mueller a moniliformis , and 
Hecastophyllum Brownii , form dense thickets and extend as 
far from the bank as they dare. In front of these is an advance 
guard of mocca-mocca ( Montrichardia arborescens) , which is as 
it were drawn up in rank to keep back the flood. Growing in 
the water, this monster arum develops great club-like stems, 
which come up as close to each other as they can pack, and 
rise like rows of palisades to the height of twelve feet or more 
above the surface. As if this were not a sufficient encroach- 
ment on the open space, the floating island grass ( Panicum 
elephantipes) anchors itself to the mocca-mocca or bushes, and 
extends just as far across as the rapid current will allow. In 
dry weather, when the water is low and the stream has little 
power, the extensions from either side meet in the centre, and 
close the passage-way for a time — only, however, to be torn 
away in great masses as the floods come. At such times great 
patches, fifty feet or more in diameter, are seen floating down 
