64 PLANT MIGRATION 
the rate of, say, 1,000 feet in half a minute (or nearly 
23 miles per hour, a good breeze), the seed will 
be carried 1,000 feet before it reaches the ground. 
Its course will be represented by the diagonal AD of 
the accompanying figure, where AB represents the 
distance which the seed falls in the given time, and 
AC the distance according to the same scale travelled 
by the wind in the same period. 
A Cc 
= = an re eee 
Baar es . 
B a uitanne re aid D 
Fic. 12, 
But most seeds sufficiently light to be capable of 
extended flights are liberated only a few feet from the 
ground; they are dependent on upward eddies to raise 
them if they are to achieve more than a very short 
migration. That such eddies, both upward and 
downward, occur on a windy day we all know from 
experience; and it is they that make or mar the for- 
tune of most wind-borne seeds. Only some local or 
accidental excess of upward over downward eddies 
will assist a seed on its journey; and as every upward 
eddy must be compensated somewhere by a downward 
eddy, the longer the journey is, the more such eddies 
tend to neutralize each other. Over the sea—that 
most formidable barrier to plant migration—eddies 
do not prevail as they do over rough ground, so that, 
unless by a series of lucky eddies a seed is whirled 
up to a considerable elevation before it leaves the 
shore, the chances of its successful passage across a 
stretch of water are remote. Discussing the possi- 
bility of seeds of Portuguese plants reaching the 
