Off THE STRUCTURE OE SEEDS. 355 



. to a part called by him the vitellus, which, though not en- 

 tirely unobserved by preceding philosophers, had received 

 no particular description or explanation. Before we enter 

 upon the investigation of this organ, it is necessary to con- 

 sider the structure and functions of the parts of a seed in 

 general ; and this it will be best to do physiologically. 



Three agents are necessary to the germination of seeds, — Germination of 

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

 absorbs, through the vessels of its base, the juices of the and air. 

 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 latter, 

 for the purposes of vegetation, than others. Moisture and 

 lieat however are not of themselves sufficient to cause the 

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

 equally necessary ; and modern chemists have ascertained 

 oxigen gas to be the particular ingredient of the atmospheric 

 air which is requisite, and which is absorbed by seeds, in 

 the moments of incipient germination, from or through the ** 

 surrounding soil. Thus the bulk of the seed is increased, 

 and its vital principle stimulated. It bursts its immediate Integument, 

 integument, or testa, and in the first place sends forth the 

 radicle, or young root, into the ground. 



This part being, as Dr. Darwin well observes, most sus- Radicle. 

 ceptible of the stimulus of moisture, elongates itself in the 

 direction in which it meets with this stimulus ; and descend- 

 ing into the earth, while it fixes the infant plant, assumes 

 its own proper function of imbibing nourishment for the fu- 

 ture support of that plant. 



But before any supplies can be thus obtained, consider- Albumen. 

 able demands are made, even by the root itself; and not 

 only an evolution of parts, but likewise an increase of bulk, 

 takes place in the young vegetable. For this necessary pur- 

 pose a store is prepared in the albumen, a substance either 

 constituting a separate body by itself, as in grasses, corn, 

 palms, &c, which, from a hard, dry, and tasteless mass, 

 changes, by the action of water and oxigen, into a milky or 

 saccharine fluid ; or the same substance is lodged in, or united 

 with, the bulk of another part, next to be mentioned, the co* 

 tyledon, or, as they are generally of the plural number, 

 cotyledons. 



2 A 2 As 



