Inquiry into the Structure of Seech. 233 



The luminous appearance of those meteors connected with 

 the fall of stones, is one of the extraordinary circumstances 

 of these wonderful phenomena. This effect may be ac- 

 counted for, by supposing that the substances which fall, 

 come into our atmosphere in a metallic state : and that the 

 earths they principally consist of are a result of combustion; 

 but this idea has not the slightest connection with their ori- 

 gin or causes. 



XXXIV. An Inquiry into the. Structure of Seeds, and espe- 

 cially into the true Nature of that Part called by Gcertner 

 the ritellus. By James Edward Smith, M.D. F.RS. 

 P.L.S* 



VJ^ERTNER, so justly celebrated for his anatomical and phy- 

 siological inquiries into the nature of seeds in general, arid 

 for his particular illustration of one thousand different kinds, 

 claims the merit of first giving a name and definition to a 

 part called by him the Vitellus, which, though not entirely 

 unobserved by preceding philosophers, had received no par- 

 ticular description nor explanation. Before we enter upon 

 the investigation of this organ, it is necessary to consider 

 the structure and functions of the parts of a seed in genera! - r 

 and this it will be best to do physiologically. 



Three agents are necessary to the germination of seeds, — 

 moisture, heat, and air. A seed committed to the ground 

 absorbs, through the vessels of its base, the juices of the 

 soil, or any other moisture that comes in its way ; while it 

 receives, throughout its whole substance, a definite portion 

 of heat, some seeds requiring a greater share of the lalter. 

 for the purposes of vegetation, than others. .Moisture and 

 heat, however, are not of themselves sufficient to cause the 

 germination of seeds. It has long been known, that air i& 

 equally necessary ; and modern chemists have ascertained 

 oxygen gas to be the particular ingredient of the atmospheric 

 air which is requisite, and which is absorbed, by seeds 3 iv.. 



* From l,inaxzvi Tries;;e:ians, vol. ix. p 3Q4» 



the 



