WATER DISPERSAL 61 
auger, and, lengthening as it untwists, forces the seed 
into the ground. Should dryness supervene, the back- 
ward-pointing hairs on the seed-envelope prevent its 
being drawn out again when retwisting and conse- 
quent shortening take place. These Erodium fruits 
are among the most interesting in the British flora, 
and are well worth experimenting with. 
2. Water.—Water, which forms the most frequent 
and the most serious barrier to plant migration, under 
certain circumstances is a very efficient agent of 
dispersal. At the same time, its powers in the latter 
direction are strictly circumscribed. As regards fresh 
water, seeds which float may be wafted across lakes. 
Rivers are more effectual, as seeds may be trans- 
ported long distances in their currents and thrown up 
finally on their banks or over flooded areas. When 
we consider the sea, we realize that there is here a 
possibility of almost unlimited dispersal provided that 
the seeds are not injured by salt water, and that they 
can remain afloat. It is on the latter point that the 
whole efficacy of water dispersal turns. This was 
long ago recognized, and investigations have been 
made by many naturalists to determine the buoyancy 
of seeds of all kinds. The results show that, taking 
the seeds of the plants of any country as a whole, not 
more than about Io per cent. are capable of floating 
for more than a short period, while most of them sink 
at once in either fresh or salt water. So one’s vision 
of seeds transported in myriads over hundreds of 
miles of sea is rudely dispelled; and the fact that many 
seeds can survive prolonged immersion in sea-water 
uninjured is of little account. The to per cent. of our 
own flora which produce buoyant seeds are mainly 
