66 PLANT MIGRATION 
rate of fall, and a consequent power of long-distance 
dispersal by wind. For instance, the microscopic 
spore of the puff-ball Lycoperdon falls so slowly 
that, if we take again Guppy’s Azores example, it 
could traverse the 800 miles in a 50 miles an hour gale 
if it commenced its flight only 86 feet above the 
ground. Such spores are, in fact, so buoyant that 
they form a normal constituent of the air—as we 
know, for instance, by the rapidity with which they 
will discover and germinate upon a piece of cheese, 
forming bluemould—and with little doubt they are 
capable of reaching under favourable circumstances 
the most distant of oceanic islands. But in the Flower- 
ing Plants with which we are mainly concerned re- 
duction in size is not carried far enough to confer 
any great amount of buoyancy. The minute seeds of 
the Poppies (Papaver), for instance, fall about 10 feet 
in a second. Applying again Guppy’s Azorean case, 
we find that though these would cover the distance 
in sixteen hours, they would fall in that time about 100 
miles, unless raised during the journey to that ex- 
tent by the excess of upward eddies as compared with 
downward ones—a quite impracticable proposition. 
In the Orchids alone do we find among the powder- 
seeded Flowering Plants a really effective buoyancy; 
this is due to the fact that great reduction in size is 
accompanied by very loosely disposed tissue enclos- 
ing the seed in a kind of net, and by the resistance to 
the air thus offered, greatly reducing the rate of fall. 
The seed of the Marsh Helleborine (Epipactis longi- 
folia) falls only about 7s as fast as that of the Poppies, 
and would thus, under the same conditions, be carried 
fifteen times as far. 
