STRUCTURE AND HISTORY OF PLAY. 259 
pressure, and therefore the greater the pull on the rooted portion ; thus 
aggregations of tussocks solidified above by earth and by accompanying 
plants are more likely to become detached than smaller tussocks. I do not 
think that water-pressure caused by floods is absolutely necessary in order 
that detachment of the reed should take place. It may in water as deep as 
that of the delta become detached merely owing to the death of its basal 
rhizomes, though detachment must naturally take place most readily under 
conditions which tend towards instability, that is, during floods and storms *. 
Were floods the master-factor as regards the detachment of the reed, 
there would hardly be any rooted reed in the delta, since floods occur 
generally twice in the year, whereas in general reed of a more or less definite 
stage of development becomes detached, and yet reed-swamp is far in excess of 
Play. In fact, I do not regard a soft bottom and floods as the essentials 
of Piav formation (see p. 244). 
That the detachment of the reed is not, however, merely due to its 
strueture and age, but also to some extent to its environment, is shown by 
the facts (1) that there is practically no Plav beyond the delta, (2) that 
Plav does not arise everywhere in the delta, bat only in deep water, and 
(3) that in the Norfolk Broads no Plav is formed, though the reed forming 
the fen has a distinet basal decomposition-layer. 
The absence of Plav beyond the delta is, [ think, attributable to the 
magnitude of the floods, which carry a very much greater amount of fine 
inorganic silt, a large amount of which is finally deposited in the quiet 
reaches of the Balta. The reed, however insecure it may become through 
the processes of its growth, is constantly being planted afresh through 
the deposition of this silt. Thus around Tulcea, in the Somova plexus 
of lakes, very little Plav (the Prundoae of the local fishermen) is formed (see 
p. 245), and higher up the river where the deposition of silt is still greater, 
near Bráila for example, I have not seen even Prundoae. The reed there 
grows on а ledge of silt because the quantity of silt is so great that 
practically wherever there are plants, а bank is formed extremely rapidly. 
Many of the reed-thickets around Bräila can be crossed on foot dryshod. 
That Plav does not arise in shallow water around the grinds is due 
apparently to the smallness of the space under water: the reed therefore 
fills it completely. Hence it is far better secured than in deep water, and 
forms a most compact platform similar to Plav in structure although, unlike 
it, it is attached to the bottom. The water is often only a few centimetres 
deep above these platforms though they look like the ordinary swamp, or, 
rather, are liable to be taken for such by anyone unacquainted with the 
configuration of the ground. In time the space between the surface of these 
* The Typha angustifolia around the edge of Horsey Mere floats because its basal parts 
have died. The rise and fall of the water in Horsey Mere is more or less irregular. 
