STRUCTURE AND HISTORY OF PLAV. 257 
full-grown reed, and in Lake Rosulet, two fairly large sized reed-islands 
with a rim of open reed-swamp outside and a Plav kernel within *. 
Open reed-swamp is of different widths. This difference of width of the 
open stage of growth would appear to follow as a matter of course in any 
plant that advances concentrically. The radial advance of reed spreading 
from a centre must inevitably diminish even though the vegetative output of 
the reed be on the increase, because the circumference of the successive 
circles increases—the fresh area covered each year may be greater and yet 
the radial advance less. If we suppose that the fresh area acquired annually 
be the same, the radial advance will decrease steadily, and may finally be so 
slight as hardly to be perceptible, in which case closed reed-swamp will in 
time abut on the open water and the reed-swamp appear to be stationary. 
Naturally physical obstacles such аз too great depth of water t will stop the 
reed from advancing, in which case also closed reed-swamp will in time 
abut on the open water. Even minor inequalities of the bottom influence 
the direction and manner of growth of the reed. Thus the closed reed-swamp 
stage wil appear in some spots sooner than in others. The annual incre- 
ment of the reed is certainly on the increase as long as the reed is advancing 
horizontally, because not only does the reed acquire fresh territory each 
year, but fresh shoots also arise each year all over the area already 
oceupied ; in fact, the shoot output at first, at any rate, increases behind the 
vanguard. There appears to be no decrease in the annual increment until 
close on the time when the reed ceases to advance horizontally and becomes 
detached, i.e. Plav. The reed, about the time of its detachment, produces, 
per annum, per 4 square metres (6 feet 62 inches across or about 43 square 
feet), about 50 to 60 shoots which reach the air, their weight (green) being 
from 14 to 17 lbs. (about 6°8 to 7^7 kilogrammes) (see pp. 265 & 268-269). 
It 50 shoots reach the air, then there must be 50 new pieces of rhizome, 
since the portion of shoot in the air is the continuation of the rhizome (see 
p. 248). These rhizomes do not, however, all arise from the base—some are 
relatively short (see p. 207). Two basal rhizomes taken from a swamp weighed 
(green) about 2 Ib. (about 340 grammes), each. Therefore, allowing 10 basal 
rhizomes per 4 square metres (43 sq. ft.) at $ lb. (about 225 grammes) each, 
and 40 at 1 Ib. (about 113 grammes), the output of rhizome per annum рег 
4 square metres is at least 15 lbs. (about 6:8 kilog.). 
Now the extremities of shoots of three years’ standing often occur at 
the surface (see p. 248), and as their rhizome portion is perennial, even 
though they themselves are annual, the total weight of rhizome per 
4 square metres (43 square feet) is at least 45 lbs. (about 20:4 kilogs.), the 
* [ sounded the larger of these islands and found a shoal. 
t I have found reed growing in water up to 2:8 m, (about 9 feet 6 inches) deep at high 
water in the Danube. 
