230 Inquiry into the Structure of Seeds. 



more especially, confirm this, by teaching us that carbomc- 

 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 oxvgen emitted by the under sur- 

 face. 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 pro- 

 cess, 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, 

 wltt, albuminous matter aboundine; witlrthe carbonic prin- 

 ciple. The chemical action of the oxygen on this albumi- 

 nous substance, renders the latter a more or less saccharine, 

 and, with the addition of the imbibed moisture, a milky 

 fluid, fit to be transmitted, through the returning vessels of 

 the cotyledons, into the stem of the embryo, especially as 

 all these important pairs 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, thai is ever positively black, except perhaps 

 the skins of some fruits. It is, moreover, evident that all 

 the indispensable functions of the cotyledons are best per- 

 formed under ground, and that when they rise into the air 

 and light, it is not till after their primary destination is ful- 

 filled, and then because, being fundamentally of the nature 

 of leaves, they are also capable, inmost instances, of as- 

 suming their functions with respect to light. It is highly 

 worthy of notice that, in consequence of the original posi- 

 tion of the cotyledons in all seeds, the oxygen gas must al- 

 ways be in;bibed by their under side, that very same part 

 which in leaves gives out this kind of gas during the day, 

 and probably absorbs it during the night. It would have 

 evinced a strange counariety in the constitutions of two or- 

 gans otherwise so analogous, I mean the leaves and cotyle- 

 dons, if the upper surface of the latter, while in the uneoj- 



panded 



