VITALITY OF BURIED VEGETABLE GERMS. 251 



sod in which the ordinary annuals are unable to nourish. 

 Break up the sod after any number of years, and subdue the 

 perennial grasses, and we shall have a crop of annuals the 

 first season — Veronicas, Chenopodiums, Euphorbias, Por- 

 tulacas, Ambrosias, Crab -grasses, Foxtails, Panicums, etc. 

 Cease cultivation, and the Poas and Glycerias will imme- 

 diately resume possession. Similarly, the pertinacity with 

 which the common knot-grass seizes and maintains its po- 

 sition only along the hardest-beaten footpaths is notorious, 

 Avhile the greater plantain renders itself no less conspicu- 

 ous growing alongside. Earth thrown out of cellars and 

 wells is generally known to send up a ready crop of* weeds, 

 and, not unfrequently, of species previously unknown in 

 that spot. In all these cases, after allowing for all known 

 possibilities of the distribution of seeds by winds, birds, 

 and waters, it still seems probable that germs must have 

 previously existed in the soil. 



Similar sudden appearances of new forms take place 

 when a change is effected in the chemical nature of the 

 soil. Illustrations are familiar to every agriculturist. 

 How soon does a dressing of undecomposed muck, or 

 peat, or sawdust develop a crop of acid-loving sorrel, and 

 how readily is it again repressed by a dressing of some al- 

 kaline manure ? Let the waters of a brine-well saturate a 

 meadow, and how long before we witness the appearance 

 of the maritime Scirpus and Triglochin, or some other salt- 

 loving plant whose germs, unless spontaneously developed, 

 must have lain dormant at a greater or less depth ? 



Something of the same nature is witnessed on the disap- 

 pearance of dominant species, whether through the agency 

 of man or the processes of Nature. It is well known that 

 the clearing of a piece of forest and the burning of the 

 brush is almost always followed by the appearance of cer- 

 tain unwonted plants known as "fire-weeds." In many 



