VITALITY OF SEEDS. 995 
spring up in its place; and when they, in their turn, fall be- 
fore the axe, sometimes even as soon as they have spread their 
protecting shade over the surface, the germs which their prede- 
cessors had shed years, perhaps centuries before, sprout up, and 
in due time, if not choked by other trees belonging to a later 
stage in the order of natural succession, restore again the origi- 
nal wood. In these eases, the seeds of the new crop may have 
been brought by the wind, by birds, by quadrupeds, or by other 
causes ; but, in many instances, this explanation is not probable. 
When newly cleared ground is burnt over in the United 
States, the ashes are hardly cold before they are covered with 
a crop of fire-weed, Senecio hieracifolius, a tall, herbaceous 
plant, very seldom seen growing under other circumstances, and 
often not to be found for a distance of many miles from the 
clearing. Its seeds, whether the fruit of an ancient vegetation 
or newly sown by winds or birds, require either a quickening 
by a heat which raises to a certain high point the temperature 
of the stratum where they lie buried, or a special pabulum fur- 
nished only by the combustion of the vegetable remains that 
cover the ground in the woods. 
Earth brought up from wells or other excavations soon pro 
duces a harvest of plants often very unlike those of the local 
flora, and Hayden informs us that on our great Western desert 
plains, “ wherever the earth is broken up, the wild sun-flower 
(Helianthus) and others of the taller-growing plants, though 
previously unknown in the vicinity, at once spring up, almost 
as if spontaneous generation had taken place.” * 
Moritz Wagner, as quoted by Wittwer,t remarks in his de- 
scription of Mount Ararat: “ A singular phenomenon to which 
my guide drew my attention is the appearance of several 
plants on the earth-heaps left by the last catastrophe [an earth- 
quake], which grow nowhere else on the mountain, and had 
never been observed in this region before. The seeds of these 
plants were probably brought by birds, and found in the loose, 
* Geological Survey of Wyoming, p. 455. 
+ Physikaiische Geographie, p. 486. 
