DISPERSAL BY WATER 69 
the land. In still waters we can find many that float 
on the surface, although, as a rule, only for a com- 
paratively short time, and while floating they may be 
blown by the wind for a considerable distance. It is 
interesting to collect a handful of débris from the 
margin of a pond, lake or river, if not altogether 
easy to supply the names of the plants to which the 
seeds that it contains belong: a good deal, however, 
can be learnt by making the attempt, and knowledge 
so acquired has the great merit of being very apt to 
stick. 
A gentle wind or a breeze over a lake will effect 
a good deal of dispersal for such seeds as float, while a 
gale which lashes its waters into foam and spray will 
carry comparatively large ones a considerable distance 
beyond its margin, though, of course, by no means 
always to a suitable spot. 
(c) Dispersal by Water 
In the examples that we have just noticed it is 
the wind and not the water that is the motive force, 
but water is by no means to be despised as an agent 
by any one who studies dispersal. There is not very 
much to be said about it within the limits of this book, 
which do not include dispersal by ocean currents 
over hundreds or thousands of miles; but it plays a 
very considerable part in short-distance and _ local 
dispersal within the limits of our own islands. 
Running water, for example, is capable of much 
greater things than the wind blowing over a pond 
or lake, and the seeds of some plants are carried long 
distances by streams and rivers. I referred just now 
to the Yellow Flag, which is common enough to be 
very well known. Its large light seeds are capable of 
