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164 NOTES ON PONDWEEDS. 
Pedunceles shorter than the leaves at their base, often produced in 
clusters, stout, swollen upwards, regularly curved from the base, 
terminal, or lateral only from the fertile branchlets lengthening and 
Nowering ; spike short, dense, cylindrical, 1-14 in. long. Drupelets 
staall, flattened at the sides; dorsal margin blunt, with prominent 
keel and ridges, or acutely keeled; upper margin straight, ter- 
minated by the short beak: variable. Colour of the whole plant, 
green or olive-gree 
stinguished by its regularly curved peduncles, which, together 
with the fruiting spikes, are shorter than the leaves from whence 
floating leaves are pres o species are still more easily 
separated. In P. heterophyllus the petiole of these leaves is almost 
invariably longer than t mina, whil P. Zizi it is much 
shorter, or rarely equalling the lamina. Out of many hundreds of 
leaves carefully examined, I have met with only one lea of P. Zizii 
liable to som 
gree of variation, the result of unusual or irregular conditions of 
growth—accidental states which a little experience will soon enable 
the careful observer to detect and explain. In P. heterophyllus the 
peduncles are usually terminal, or only become lateral by the con- 
tinued growth and flowering of the fertile branchlets. In P. Zizii, 
on the other hand, they become lateral by a barren branchlet 
throughout its entire existence a uniform growth, resembling that 
of the lower branches, especially when growing in deeper or colder 
water than usual ; or, perhaps, when growing in a more northern 
latitude; and that the reverse is the case in shallow or warm 
ters. Many continental authors, probably founding upon some 
such local races, have ivided P. heterophyllus into two sections, 
which, however, are not sufficiently well represented in the fens to 
