FLOEA OF THE AVON-BED. 
113 
posing that the seeds lay at the bottom of the ditches preserving 
their vegetative power until a favourable opportunity offered for 
them to grow, when cast out with the mud. It has also been 
noticed by observers in other parts of the w^orld when new 
soil is turned up, excavations made, and forests burned or other- 
wise cleared, that plants previously unknovrn in the locality 
spontaneously appear, which is a proof that the seeds of such 
plants must have lain dormant for very lengthened periods. 
Among the conditions requisite for germination is a free 
supply of air, or at least of oxygen ; so that seeds cannot 
germinate when buried deeply in the soil, or when that soil is 
impervious to the air, and this explains how it is that seeds may 
lie quiescent at great depths, and only germinate when brought 
to the surface. 
Positive evidence from direct experiment is not wanting to a 
certain extent, although the inferior longevity of man forbids the 
supply of exact proof. However, seeds of plants preserved in 
herbaria have germinated after more than a century had elapsed 
since they w^ere collected ; and the seeds of Leguminosse especi- 
ally have been observed to retain their vitality for many years. 
That natural order is well represented in the list of plants 
mentioned in this paper. 
From the evidence that has been advanced, it seems clear 
that all the plants under notice sprung from seeds buried in, 
and dredged up with, the Avon mud. We will now endeavour 
to ascertain the sources from which these seeds originally came. 
As regards the native species whose representatives still hold 
their ground on the banks of the Avon, it can readily be under- 
stood that many of their seeds must necessarily be shed upon 
the stream, washed to and fro by the tide, and eventually be 
entangled and buried amid the shifting mud-banks. 
Greater interest attaches to that section which represents 
species formerly found in the Avon valley, hut of late years 
