KEEPING WARM 
in its distribution — from San Francisco 
Peaks to the high volcanoes of Ecua- 
dor — the species extends southward 
down the Andes to Cape Horn. Mulroy 
found that the South American form, 
which has been called S. magellanica, 
differs little from S. cespitosa of the 
Northern Hemisphere. She suggests 
that some of the species’ extension down 
the Rocky Mountains could have been 
the result of island-hopping by birds and 
that other, shorter movements could 
have taken place during full glacials. 
But the long gap from Arizona to the 
equatorial mountains and the presence 
of S. cespitosa throughout the alpine 
and subantarctic regions of South 
America south of the Equator make it 
almost impossible not to accept one or 
more events of long-distance dispersal 
by birds. Mulroy suggests that alpine 
Peru may have been the site of S. 
cespitosa's original introduction to 
South America, but since some of the 
wading birds near Point Barrow, Alaska, 
fly to Argentina for the Southern Hemi- 
sphere summer, there is also the possi- 
bility that Saxifraga seeds could have 
been carried there and that the species 
has since moved northward along the 
Andes. Since the Southern Hemisphere 
S. cespitosa plants differ in small ways 
from those of the Northern Hemi- 
sphere, any long-distance dispersal prob- 
ably occurred some time ago, but there 
is nothing to prevent such an event from 
taking place now. 
Studies of present-day distributions of 
Moss campion roots get moisture from 
deep in the soil. Water availability 
and a plant's ability to obtain it 
or get along with less are two of the 
factors that influence vegetation 
patterns in alpine and arctic areas. 
Martin W Grosnick. AlaskaPhoto 
various arctic and alpine species have 
led to a greater understanding of general 
biogeographical patterns for cold-cli- 
mate plants. Most of the ancestors of 
today’s alpine plants appear to have 
been desert and grassland species that 
migrated and evolved “up-mountain,” 
especially during the Pleistocene, a pe- 
riod of mountain-building. There is a 
tendency for alpine floras to show much 
endemism, that is, to be unique to a 
given mountain range or mountain sys- 
tem. Once a species has adapted to its 
particular kind of fluctuating cold envi- 
ronment, there seems to be little back- 
migration down the mountain to the 
drier or more forested environments. 
For these cold-adapted, light-requiring, 
but not drought-resistant species, mi- 
grating to other high mountain ranges or 
the Arctic is easier. Once in the Arctic, 
becoming circumpolar or almost so is 
also relatively easy. Under the right 
climatic and topographic situations, 
some of these circumpolar species mi- 
grate southward again on the mountains 
of entirely different continents. Lack of 
fossils always makes it difficult to say 
where a given widespread species origi- 
nated; finding out, by experiment, why 
and how the species maintains itself 
where it is currently found is much 
simpler. Nevertheless, my feeling is that 
Oxyria originated in the subalpine re- 
gions of the mountains of southwestern 
China, where its only closely related 
relative occurs. 
Certainly, many other arctic-alpine 
species originated in the same way, by 
migrating up the mountain ranges of 
Eurasia or North America. There are 
some truly arctic species in unique arc- 
tic environments, but determining how 
most of these species originated is diffi- 
cult. The Arctic has had many changes 
in climate, and at times parts of it may 
have had a forest cover. A good portion 
of the endemic arctic vascular plant 
flora may have evolved in place as cli- 
mates changed and forest cover re- 
treated southward, but at this time, we 
can only hypothesize. □ 
88 
