ON VKGETABLE PHYSIOLOGY. 83 



admission of light is a great bar to the process, 

 and therefore covering the seed with soil is a 

 necessary practice. 



Thus the external circumstances essential to 

 germination, are water, a certain degree of heat, 

 atmospherical air, the exclusion of light, and in 

 general, (though not always,) the agency of the 

 earth. 



Having made you acquainted with the structure 

 of the seed, and the requisite circumstances to 

 produce vegetation, the succeeding process ad- 

 mits of easy explanation. 



It has been previously observed, that most 

 seeds have one or more cotyledons or lobes, con- 

 taining a quantity of farinaceous matter laid up 

 on purpose to supply the embryo plant with food 

 as soon as it is required. This food, however, 

 must undergo some previous preparation, before 

 it can be taken up by the plant for the formation 

 and completion of its respective organs ; and 

 such an effect, it is obvious, can only be produced 

 by chemical agency. When a seed therefore is 

 committed to the earth at any temperature above 

 forty degrees, it imbibes water, which softens and 

 swells the cotyledons, and decomposing the 

 external coat, it admits the absorption of oxygen 

 from the atmosphere, which combining with the 

 carbon in the seed, carbonic acid gas is generated 

 and set at liberty. This loss of carbon increases 

 the proportion of oxygen and hydrogen in the 



G 2 



