CAUSES. 



situation never continued to grow. The experiments 

 of Homberg have been repeated with great fidelity by 

 others, who infer that in the cases of germination men- 

 tioned by him the vacuum must have been very in- 

 complete. Seeds which have remained for years 

 unchanged buried deep in the earth, are often seen to 

 germinate when raised within the influence of the at- 

 mosphere ; and the chesnut and acorn are found to 

 grow better on the surface of the earth than when 

 buried beneath it. Achard proved that seeds grow 

 more luxuriantly in compressed than in rarefied air, 

 and the successors of the illustrious Scheele, have by 

 a series of ingenious experiments ascertained that it is 

 the oxygen of the atmosphere which excites their ger- 

 mination. 



Water and heat are also agents of primary impor- 

 tance. The arid sands of Africa are scarcely embel- 

 lished by a single plant, and the cold of our winter 

 most effectually prevents the germination of the seeds 

 which are scattered over the earth ; but let the rain 

 descend and it will cause the desert to blossom as the 

 rose, and when spring returns diffusing cheerfulness 

 and warmth, it will revive the energies which slumber 

 within the coats of the seed. 



The moisture which causes the first enlargement of 

 the seed is chiefly absorbed through the scar (a small 

 point by which it was originally connected with the 

 fruit, and easily discernible on the surface of most 

 seerls) but it is also imbibed by the integuments, for 

 the entire exclusion of water from the scar, retards but 

 does not arrest the process of germination. In either 

 case the water which is absorbed passes through the 

 vessels of the cotyledons to the rostel of the infant plant, 

 which exhibits the first indications of vitality. Hence 



