STRUCTURE AND HISTORY OF PLAV. 267 
apparently, are wholly dependent on the shelter from the climate afforded 
them by the reed. Thus, when the reed dies and the shelter is reduced, 
although the accompanying vegetation remains in possession of the Plav, 
its constitution changes (see рр. 270-271). There is no steady increase in 
number of the accompanying plants as the reed ages, only a change of 
species ; in fact, the accompanying plants appear to decrease in number, 
judging by the weight (see No. IV. of the Table on р. 265), though, since 
the species differ to some extent, the weights are not strictly comparable. 
Further, there is no root competition with the reed, since the accompanying 
plants are all superficially rooted. The reed is obviously a true dominant, 
the accompanying plants being dependent on its presence, whilst it is wholly 
dissociated from theirs. 
The sum of the facts given with regard to the minor individual, the reed- 
shoot, leads, it seems to me, inevitably to the conclusion that the decrease in 
size of the reed-shoots is inherent to the constitution of the reed major 
individual 
registers the passage of time as regards the reed major unit; or, in other 
in fact that it is a progressive morphological change which 
words, that it is a measure of its absolute age—an indication of the point 
reached by the reed in the running of its predestined course the end of 
which is death. On this explanation the giant shoots are those which have 
arisen earliest and at the base of the branch-system *, and the slender ones 
such as have spruug from it later and higher up, whilst the shortest slender 
shoots are highest up in the system. As already explained, however, short 
shoots do not necessarily in themselves indicate loss of vitality (see Table on 
p. 265) in the major unit, but they herald it t. 
The decline in size of the reed-shoots apparently follows merely as a result 
of the increase in compoundness of the reed-system in the vertical direction. 
The tallest shoots of a Plav nearly always arise from the base ; they are the 
lower branches of the reed-system, and therefore less compound with refer- 
ence to the original basal rhizome than their fellows ; and this constitutes 
* [ would suggest, tentatively, that the sequence of shoots in plants is preordered or 
morphological, and therefore more or less definite—that is to say, is essentially independent 
of nutrition. The beech, for example, produces full crops of good seed in England, generally 
from 50 or 60 years of age onwards. I would suggest that this is not owing to the accumu- 
lation of nutritive substances, but because the beech takes 50 to 60 years, under a normal 
output of branches, to reach the stage in its life-cycle at which branches bearing numerous 
flowers appear: the plant must produce a more or less fixed quantity of branches of one 
kind before it can produce the more fertile branches of the next stage, see p. 253. 
1 As regards the possibility of internal factors such as decrease in the efficiency of the 
absorbing and conducting organs, or the presence of internal toxins (see Benedict's 1915 
paper, pp. 325-830) causing the change in size of the shoots, I can only repeat what I said 
on pages 264-266, viz., that the vigour of appearance and of output of the reed is unimpaired 
for a considerable period after it has entered on the slender-shoot stage, which would be 
unlikely were it suffering from lack of food, or were it becoming poisoned. 
тд 
