248 BOTANICAL GAZETTE [OCTOBER 
eximia DC. is another illustration, growing in a rather restricted 
area in the Doe River Gorge, where the river cuts between Iron 
Mountain and Gap Creek Mountain, North Carolina. Shortia 
galacifolia Torr. & Gray, Lilium Grayi S. Wats. on Roan Moun- 
tain, Buckleya distichophylla Torr. on Paint Rock are examples of 
this same local distribution and isolation. 
PHENOLOGIC DISTRIBUTION OF PLANTS. 
Four kinds of plants with reference to their phenologic dis- 
tribution may be distinguished in the vegetation of the forests of 
eastern North America; viz.: plants of boreal genera (Arctic, 
Hudsonian, Canadian species), plants of temperate genera 
(Alleghanian and Carolinian), plants of warmer temperate cli- 
mate (austro-riparian), and neotropic genera. 
Both boreal and temperate species bloom in the spring before 
they are shaded by the leafing trees, but for different reasons. 
The plants of temperate origin vegetate and blossom in the 
spring before the trees are in leaf, because, as a matter of light 
relationship, it is the only season in which the functions of these 
plants can be performed adequately, and this presupposes the 
influence of a certain number of heat units greater than for 
boreal species and less than for those of more southern origin. 
A more detailed statement on this subject will be made subse- 
quently. The boreal plants, however, of the eastern Appala- 
chian forests, blossom and vegetate early in the season, because 
that is the part of the year at which the development of these 
species, which require a minimum of heat units, is most ade- 
quately performed.*? The writer has shown this in a detailed 
manner in a paper published in Science.* The Scandinavian 
element of our flora consists of plants which mature their seeds 
quickly before the summer is well advanced. This marks them 
sence of many boreal plants in the forests and peat bogs of the northern 
states is accounted for by the encroachment of tree vegetation, after the glacial 
epoch, upon the areas occupied by the boreal plants, which surrounded by the trees, 
were destroyed or compelled to grow during the period when the trees are leafless. 
he movement of trees northward was occasioned by the fact that the land area, left 
bare by the retreat of the glaciers, was one of low tension, while the country to the 
south, as we have seen, was one of high tension. 
™ HARSHBERGER, The origin of our vernal flora. Science N.S. 1:92-98. Ja 
25. 1895. 
