36 POPULAE EREOES. 



which forms the casts on the surface; and though 

 to common observation no seeds are apparent, 

 whenever the bisected pipes or chambers happen to 

 remain exposed to the weather on the face of the 

 hard clay section for a sufficient time the whole 

 becomes green from the growth of grasses, the seeds 

 or germs of which must apparently have existed in 

 the fine black earth." 



Darwin, in his remarkable work on the forma- 

 tion of vegetable mold by means of earth worms, 

 gives similar examples of seeds being carried into 

 the soil by their agency. 



Regarding the duration of the seeds of forest 

 trees in the soil, Henry D. Thoreau makes the fol- 

 lowing remarks in an article on the succession of 

 forest trees in the report of the Massachusetts 

 Board of Agriculture for 1860: " So far from the 

 seed having lain dormant in the soil since oaks 

 grew there before, as many believe, it is well known 

 that it is difficult to preserve the vitality of acorns 

 long enough to transport them to Europe; and it 

 is recommended in Loudon's Arboretum, as the 

 safest course, to sprout them in pots on the voyage. 

 The same authority states that very few acorns of 

 any species will germinate after having been kept 

 a year, that beech mast only retains its vital prop- 

 erties one year, and the black walnut seldom more 

 than six months after it has ripened. "I have 



