STRUCTURE AND HISTORY OF PLAV. 271 
In Rumania, in the steppe region, which bounds the delta of the Danube, 
the rainfall is low and evaporation high, hence there is a tendency for salt 
to appear: consequently, the edge of many of the grinds in the delta is 
a zone of salt-marsh ; in fact, on the island of Letei, the largest grind in 
the northern portion of the delta, the village commons are in some cases 
a splendid crimson in the autumn, owing to the abundance of Salicornia 
herbacea, Linn., and Sueda maritima, Dumort. The vertical zoning prac- 
tically everywhere in the delta and in the steppe, is—fresh water in deep 
depressions, followed at a higher level, i.e. at the evaporation level, by 
salt water, and above that by a zone with practically no water. Briefly the 
vegetation succession is Hydrophyte, Halophyte, Xerophyte. 
On Plav, the succession is apparently more or less the same as in the delta 
in general. After the tall dominants have disappeared, the steppe climate 
comes into direct contact with the low-growing herbaceous plants, with the 
result that salt-marsh plants begin to appear amongst the fresh-water marsh 
plants: it seems therefore probable that climatic salt-marsh will finally succeed 
the reed and Cladium on Plav*. In Appendix D, р. 279, a list is given 
of the salt-marsh plants gathered on Ivan-Mekitenko-Plav, probably the 
oldest Plav I examined, from a spot where Phragmites was absent, and 
Cladium, which was present on one side of the Plav, had not yet spread. 
The salt-marsh plants are, however, not yet abundant. 
For lists of the plants of the delta of the Danube and of the Norfolk 
Broads, see Appendix D. The succession of the species and of the vegeta- 
tion in the delta of the Danube and in East Anglia is summarized оп 
pages 272-273. 
SUMMARY. 
1. Plav is а floating fen confined in Rumania to the delta of the Danube. 
It is formed of reed, Phragmites communis, Trin., В. flavescens, Gren. & Godr. 
2. Plav is built up almost entirely of vertical reed-rhizomes which, with 
the aid of their roots, retain much soil. 
3. The detachment of the reed, that is the formation of Plav, takes 
place at a more or less definite stage of growth of the reed. Essential 
factors to Plav-formation are :— 
(a) That the death of the basal rhizomes of the reed coincide with 
the swamp-stage of its growth ; 
* The sub-dominant plant, in fact practically the only accompanying plant of “ giant ”? 
reed Plav, û. е. newly risen Plav, is Polystichum Thelypteris, Roth. On drier Plav, the 
accompanying plants are mixed, though Polystichum is still sub-dominant (see the table on 
р. 265). Mosses occur only locally on Plav, which, in this respect, presents a contrast to 
fen where mosses abound. The only orchid I saw was Epipactis palustris, Crantz, and 
I only found it on newly risen Plav twice or three times. The absence of Polystichum from 
some Plavs, and the rarity of mosses and orchids, is possibly connected with the ease with 
which salt makes its appearance. 
