260 MISS MARIETTA PALLIS ON THE 
platforms and high-water-level becomes filled owing to the activity of aquatic 
and land plants (see Pl. 24, section IIIJ, and a land-surface is formed. 
No Plav is formed in the Norfolk Broads because the common Norfolk 
variety of reed, which invades water not more than about 1:2 m. (about 
4 feet) in depth, manages to fill the water-space completely before basal 
decomposition is at all general; that is to say, decay does not generally set in 
in the swamp-stage, but in the fen-stage. A reed-fen which I sectioned with 
the rizak close to Sutton Broad Laboratory, had a regular decomposition- 
layer at the base (see Pl. 24, section IV.). Four vertical cuts were made in 
this fen * with the rizak, and the resulting block was then lifted out with 
very little effort. The little resistance offered was due to the presence of a 
few long mud-roots which proceeded from the upper living reed into the 
soft fen soil below. Werea decomposition-layer present in swamp-reed, it 
would seem that it must float, in fact become Plav, just as some of the 
Typha around Horsey Mere does. 
Plav. 
Plav, when newly detached, does not differ from closed reed-swamp except 
in that it floats. In time, however, its resemblance to closed reed-swamp 
becomes less: the sub-aquatie layer of rhizomes, rootlets, and soil becomes 
more consolidated ; the soil darker : the top layer of soil contributed by the 
accompanying plants thicker ; and the reed-shoots, the minor individuals, 
decrease in size. Plav, in fact, is a dense floating thicket of reed, the shoots 
of which rise 1:2 m. to 5:15 m. (about 4 to about 17 feet) above its surface, 
with a шоге or less dense undergrowth of accompanying herbaceous plants 
(see Pls. 18-21). 
That Plav floats is not obvious to the eye, but to the fisherman of the delta 
it is a matter of common knowledge. It is necessary to section it, or, better 
still, to observe it in relation to the rise and fall of the water in order to 
realize this fact. 
It is possible to demonstrate that it floats in various ways. For instance, 
if two parallel cuts be carried inwards from the edge of a Plav, and a series 
of cross cuts be made between, a number of free floating blocks are liberated. 
This demonstration applies, however, in partieular to the edge of the Plav, 
not necessarily to the whole. "That it floats as а whole becomes obvious 
during the floods, for it never becomes flooded—it retains the same relative 
position to the water-level. 
The movement of the Plav is vertical only, as Plav, if not attached, is 
prevented from moving horizontally. These remarks naturally apply only if 
* Sehwimmende Inseln," which move horizontally (see below, and p. 243), 
* The contemporary fen-layer, that is to say, the root-layer formed of the parts of living 
plants, is in some places 3 to 4 feet in thickness. 
