54 VITAL FUNCTIONS OF PLANTS. [LECT. II. 



serves them for an indefinite period of time, when 

 under certain circumstances ; as when some of 

 the mucous-coated seeds, mustard or linseed, for 

 example, have been accidentally buried at a con- 

 siderable depth, and are again, after many years, 

 thrown upon or near the surface of the ground. 

 In this case, these seeds readily germinate, and 

 become as vigorous plants as if they had been the 

 produce of the foregoing year. It is owing to this 

 circumstance, that often, when forests are cleared 

 of the overshadowing trees, which had long pre- 

 vented the vegetation of any seeds in the ground 

 below them ; or when buildings are razed, and the 

 earth at their foundations turned up, plants sud- 

 denly appear, different from any in the immediate 

 neighbourhood, and even unknown by the oldest 

 inhabitants of the spot. If a seed, however, be 

 once exposed to the action of heat, air, and mois- 

 ture, in a situation adapted for its growth, it ger- 

 minates, and its vegetation cannot then be stopped 

 without destroying its vitality, and the seed of 

 course rots ; the continued application of the same 

 agents, which first called into action its powers of 

 life, being absolutely necessary for its future ex- 

 istence. 



After the parts of a plant have been unfolded 

 from the seed, its functions depend altogether on 

 the vital principle with which it is endowed. It 

 selects from the soil its nourishment, digests and 

 assimilates it into its proper substance, and depo- 



