42 Muhlenbergia, Volume 4 
A PROBLEM IN PLANT DISTRIBUTION 
By S. B. PARISH 
If one happens, amongst the familiar vegetation of a region, 
to find a plant known to be a member of the flora of some dis- 
tant country, without hesitation he accounts it an estray, an 
immigrant which may, or which may not, establish itself and 
become naturalized. But should the stranger belong normally 
in a near-by but distinct zone, it is viewed in a different light. 
If it grows in a spot possessing environmeutal conditions, here 
exceptional, but customary in its home, these conditions account 
for its presence. In this way we have what are aptly termed 
“plant islands” of one zone, encircled by the flora of a zone en- 
tirely distinct. But if no such ecological explanation can be 
given we note it as a curious and puzzling extension of range. 
Doubtless this is often all that can be done, but there are 
cases in which such alien plants are probably true immigrants, 
recent and introduced by human agency, although their journey 
has been but from one neighboring zone to another. It is not 
impossible that historical research may sometimes give a clue to 
such unexpected appearance of plants away from their proper 
habitat. I have in mind an instance which suggests such a so- 
lution. 
There is in the San Bernardino valley a certain tract of 
alkaline soil, something over a mile in length, and somewhat 
less in width. It is moist, and at the lowest part gives rise to a 
small stream. It is covered with Dzstzchizs, and on it grow 
Suaeda Torreyana, Nitrophila occidentalis and Hemizonia pun- 
gens, all of them common nitrophilous plants of the valley. 
But here also are found four other species of like character 
which belong norinally in adjoining phytogeographical subdi- 
visions. There are other alkaline tracts in the valley, appar- 
ently entirely similar to this one, and otherwise inhabited by 
the same plants, but these four aberrant ones are to be found 
here only. 
