VITALITY OF SEEDS. 287 



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 original wood. In these 

 cases, 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 gi-ound is burnt over in the United States, 

 the ashes are hardly cold before they are covered with a crop of 

 fire-weed, Seneoio hieracifolius, a tall, herbaceous plant, very sel- 

 dom 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 He bm-ied, or a special pabulmn furnished 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 unhke 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,f remarks in his descrip- 

 tion 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 earthquake], 

 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 f omid in the loose, clayey soil re- 

 maining from the streams of mud, the conditions of growth which 

 the other soil of the mountain refused them." This is probable 

 enough, but it is hardly less so that the flowing mud brought 

 them up to the influence of air and sun, from depths where a 

 previous convulsion had buried them ages before. Seeds of 



* Geological Survey of Wyoming, p. 455. 

 f Physikalische Geographie, p. 486. 



