STRUCTURE AND HISTORY ОЕ PLAV. 251 
(close оп } inch) Тһе water-roots, on the other hand, are branched to the 
third degree, almost thread-like, hardish, brown, and up to about 15 em. 
{about 6 inches) in length, and soil soon begins to accumulate on them 
(see Р]. 11). 
As the rhizomes become more and more compound, they play, together 
with the water-roots, an important part in uniting the growing reeds 
into a compact whole, their closeness increasing in proportion to 
their compoundness. ‘Thus (1) the spaces between the rhizomes become 
gradually smaller, (2) contiguous rhizome-branches become intertangled 
and bound together by the numerous compound water-roots, and (3) the 
fine soil brought by the floods, and the organic matter contributed by the 
dead remains of the reed and other plants and also animals, are retained 
by the reed-roots, and thus in time the spaces between the rhizomes are 
completely filled up (see Pls. 12-17). In this way the reed-tussock is 
gradually built up, and Plav, the aggregation of many reed-tussocks, arises. 
The reed-tussock, with its open lattice of branches below and its long, 
simple mud-roots, is comparatively loosely attached at the bottom. Circu- 
lating water can do little towards washing away the soil retained by the 
close-growing upper portion of the reed *, more especially when the accom- 
panying plants are established, but the mud overlying the sand-bottom 
where the more open-branched portion of the reed is, can, to some extent, be 
redistributed ; hence the very structure of the reed when it has reached, or 
nearly reached, full development tends to instability, and therefore also 
liability to be uprooted by floods and storms at certain stages of its growth. 
(See pp. 258 & 259.) 
Plates 11 to 17 show different stages in the development of the under- 
water portions of the reed-tussock. 
Part II. 
THE GROWTH-CYCLE OF THE REED. 
The Plant and Vegetation Units. 
The present paper, though primarily concerned with the structure and 
mode of formation of Plav, is concerned ultimately with the senile degener- 
ation and death of the reed: the conclusions arrived at as regards the latter 
question are, in fact, responsible for the manner of presentation of the 
subject-matter. Thus the term “association ” is not used here, but two 
other units which define the individual—for with senescence and death, 
the question, What constitutes the individual? is inextricably interwoven. 
The difficulty of defining the individual is obvious in the case of the reed; 
* Even protracted use of a hose on a detached reed-tussock failed to free it of its soil. 
52 
