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carbonic acid is taken away from the air ; on the 
contrary, some is added. 
Seeds are incapable of germinating, except when 
oxygene is present. In the exhausted receiver of 
the air-pump, in pure azote, in pure carbonic acid, 
when moistened they swell, but do not vegetate ; 
and if kept in these gases, lose their living powers, 
and undergo putrefaction. 
If a seed be examined before germination, it will 
be found more or less insipid, at least not sweet ; 
but after germination it is always sweet. Its co- 
agulated mucilage, or starch, is converted into 
sugar in the process ; a substance difficult of solu- 
tion is changed into one easily soluble ; and the 
sugar carried through the cells or vessels of the 
cotyledons, is the nourishment of the infant plant. 
It is easy to understand the nature of the change, 
by referring to the facts mentioned in the third 
Lecture ; and the production of carbonic acid 
renders probable the idea, that the principal che- 
mical difference between sugar and mucilage de- 
pends upon the sugar containing a larger proportion 
of the elements of water, and upon a slight dif- 
ference in the proportions of their carbon. 
The absorption of oxygene by the seed in ger- 
mination, has been compared to its absorption in 
producing the evolution of foetal life in the egg ; 
but this analogy is only remote. All animals, from 
the most to the least perfect classes, require a 
supply of oxygen.* From the moment the heart 
* The impregnated eggs of insects, and even fishes, do not 
