212 Dr. Smith's Inquiry into the Structure of Seeds. 



to the whole vegetable body. The chemical experiments, of Dr. 

 Priestley more especially, confirm this, by teaching us that car- 

 bonic-acid-gas is absorbed by leaves in the day-time through 

 their upper surface, and decomposed by them, its carbon being 

 added to the sap, and its oxygen emitted by the under surface. 

 In the dark, leaves are found to absorb oxygen. Let us apply 

 all this to the germination of seeds. The oxygen, known, as I 

 have already said, to be necessary to this process, being conveyed 

 to the seed in its dark subterraneous situation, is absorbed by its 

 Cotyledons, already stored, from the constitution of the parent 

 plant during their formation, with albuminous matter abound- 

 ing with the carbonic principle. The chemical action of the 

 oxygen on this albuminous substance, renders the latter a more 

 or less saccharine, and, with the addition of the imbibed mois- 

 ture, a milky fluid, fit to be transmitted, through the returning 

 vessels of the Cotyledons, into the stem of the Embryo, espe- 

 cially as all these important parts have already begun to swell 

 by the absorption of moisture assisted by warmth. Hence we 

 see why light is found hurtful to incipient germination, and why 

 carbonic-acid-gas may be given out by seeds at that period. We 

 perceive also why the outside of seeds is so commonly dark- 

 coloured, or even black, as in Canna, Afzelia, and others, it 

 being the only part of the vegetable body, as far as I recollect, 

 that is ever positively black, except perhaps the skins of some 

 fruits. It is, moreover, evident that all the indispensable func- 

 tions of the Cotyledons are best performed under ground, and 

 that when they rise into the air and light, it is not till after their 

 primary destination is fulfilled, and then because, being funda- 

 mentally of the nature of leaves, they are also capable, in most 

 instances, of assuming their functions with respect to light. It 

 is highly worthy of notice that, in consequence of the original 



position 



