198 SOME BRITISH PLANT GROUPS 
main difficulty induced by cold would appear to be the 
withdrawal of available water; if that goes on for too 
long, life ceases. Of course the suspension of activi- 
ties which accompanies freezing cannot continue 
indefinitely, and in the cold regions of the Earth plants 
are found only where for a sufficient portion of the 
year the maximum temperature rises above freezing- 
point enough to allow of ordinary vital functions being 
resumed. A curious point in this power of resistance 
in plants to extremes of temperature is that they 
display no obvious protective adaptations. “ Our 
present powers of investigation,” Schimper con- 
cludes,* “do not enable us to recognize in plants any 
protective means against cold. The capacity of with- 
standing intense cold is a specific property of the 
protoplasm of certain plants, and is quite unassisted 
by protective means that are external.” 
It is a far cry from the high Alps to the seashore, 
but it will be of interest to examine next the lower 
limit of the range of the Seed Plants. While the upper 
limit varies much in different latitudes, according to 
the distribution of temperature, the lower is controlled 
by sea-level, which (for our purpose at least) is 
uniform over the whole globe. The level of the fresh 
waters, whose margin marks the limit of the bulk of 
the Seed Plants, is, on the other hand, various, lakes 
being situated at different heights above (and occa- 
sionally below) sea-level, while rivers slope across the 
lands down to the ocean. While the sea margin forms 
a very real barrier to the spread of Seed Plants, the 
lakes and rivers, on the other hand, yield many 
* A. F. W. ScuimPer: ‘‘ Plant Geography’’ (English transla- 
tion, 1903), p. 41. 
