52 PLANT MIGRATION 
not suffocated by burial, sometimes even for years, in 
the soil. And while the mature plant is killed in- 
stantly by immersion in boiling water or by exposure 
to a very low temperature, some seeds, if boiled for 
a quarter of an hour, are quite uninjured, while others, 
subjected experimentally to even the temperature of 
liquid hydrogen (- 260° C., or 436 degrees of frost on 
our more familiar Fahrenheit scale), remain un- 
affected. Many seeds are liberated from the parent 
plant enclosed by or attached to appendages of 
various sorts (when they are called by the botanist 
fruits) which sometimes greatly aid dispersal, as in 
the Dandelion (Taraxacum), and sometimes appear to 
hinder it; in any case, while the young plant itself is 
usually quite small, it may, when surrounded by its 
food-supply and enclosed in its wrappings, be a bulky 
object—as is seen in the Cocoanut or Horse Chest- 
nut. In the British flora, to which we may confine 
our attention, a crab-apple (containing a number of 
seeds), a hazelnut, and an acorn (each containing a 
single seed), are the largest units of dispersal with 
which we have to deal. But these are quite excep- 
tional in size, and the average seed (using that term 
in its original sense of the natural unit of dispersal) 
- in the British flora does not exceed the size of a pin’s 
head. This remarkable reduction of size alone aids 
dispersal greatly. 
The migrations of plants are effected mainly during 
the seed stage, these tiny, tightly packed portman- 
teaux being much better fitted for travel than the 
bulky and fragile organisms to which they give rise. 
But before we consider the adventures of seeds it 
must be pointed out that a considerable, if slow, 
