262: MISS MARIETTA PALLIS ON THE 
instead of running straight up vertically, as they do further in (see Pls. 15 
& 16), straggle round and up at many different angles* (see Р]. 17). 
When the reed becomes detached it is perhaps slightly less in thickness 
than the full-grown stools of closed reed-swamp. The largest stool I 
measured was over 2 m. (about 6 feet 62 inches), and the average of seven 
full-grown ones 1:8 m. (6 feet), as was indeed to be expected, since Plav 
arises from closed reed-swamp to a great extent owing to the death of 
its basal rhizomes. The hanging dead parts must soon fall away when the 
reed is floating, since dead rhizomes are not a conspicuous feature of the 
base of a Plav but occur more or less equally throughout it. I have never 
seen a layer of dead blackened rhizomes such as Dr, Antipa mentions. 
Whether an old Plav is less thick than a new one, I cannot say. Asa 
Plav ages it wastes below; that is to say, the reed layer decreases, but there 
is a gain of soil above, the contribution of the accompanying plants. Thus 
it may well be that the thickness of a Plav, as a whole, remains more or 
less unchanged’ so long as it remains floating, but that its composition 
changes. | 
Plav has at least three distinct iayers of soil. Normally, there is (1) 
the top layer of black earthy-looking soil usually from about 6 сш. to 
about 15 em. (about 2:5 to 6 inches) in thickness, contributed by the land- 
plants; (2) a layer of fine soil of a dark brown colour whose organic content 
is about 40'7 per cent.; and (3) a basal layer of coarse brown soil with 
an organic content of about 17°6 per cent.f The fineness of the soil of 
layer (2) is probably due in part to the breaking up of the soil by the roots 
of the aecompanying plants, and also to the fact that the coarser inorganic 
matter carried by the floods has to filter through layer (3) first. I have 
found an earthworm in Plav on one occasion only—in Maximou Kut Plav. 
I did not estimate the organie content of layer (1), but its soil is almost 
black, and must therefore contain a higher proportion of organic matter 
than layer (2). The floods do not come into direct contact with this 
layer, hence its inorganie matter is probably only plant-ash (see Pl. 15 
& Pl. 24, sections I. & П.). Many long water-roots, clean because 
suspended in the water, hang from the base of the Plav (see Pl. 15 & 
Pl. 24, sections I. & IT.). Inside the Plav layer there are many mud-roots ; 
* At Salhouse Broad, in Norfolk, I found a sill of reed about l'4 m. (about 4 feet 
7 inches) wide and about 0°5 m. (about 20 inches) in thickness, which had evidently grown 
over the water which is there about 1-5 m. (about 5 feet) in depth. The structure of 
this sill was similar to that of Plav, being formed mostly of vertically running rhizomes, 
А reed-layer formed mostly of horizontal rhizomes, such as described (p. 244) and figured 
by Dr. Antipa, does not appear to exist. | 
+ The organic matter of layers (2) and (3) was estimated for me by Mr. Е. J. Farrow, to 
whom my thanks are due. Only one soil sample was examined in each case; the figures 
are, however, likely to be fairly representative, as the samples were typical in appearance, 
These three layers are quite easily distinguished in the field. 
