ORGANS OF SECRETION. 105 



analysing plants. Priestley imagined hydrogen to 

 furnish nutriment to plants, but this has been dis- 

 proved by experiment. 



The decomposition of the air in the lungs of animals 

 evolves heat, but this is less observable in the decom- 

 position of air by plants. Desfontaines found, how- 

 ever, in the cuckoo-pint, that during the formation of 

 the seed the thermometer was raised fifteen degrees 1 . 



The origin of the various odours given off by plants 

 is no better understood than that of their colours, and 

 I shall not therefore detail mere conjectures 2 . 



ORGANS OF SECRETION. 



IN animals, the liver which secretes bile, the breasts 

 which secrete milk, and the kidneys which secrete 

 urine, and similar organs, are termed glands; but 

 though we find in plants numerous peculiar sub- 

 stances somewhat analogous, we cannot always distin- 

 guish the glands that secrete these. De Candolle there- 

 fore considers every separate cell of the cellular tissue 

 already described, to act as a secreting organ, even 

 when no peculiar glandular structure can be detected. 

 The matter secreted is sometimes retained in the cells, 

 and sometimes appears on the outside as an excretion. 

 It is worthy of remark, that the secretions which are 

 poisonous, such as the juice of hemlock or of cherry 



(1) This is explained in the ALPHABET OP SCIENTIFIC CHE- 

 MISTRY, p. 108. 



(2) What is known on the subject will be given in the 

 ALPHABET OF ORGANIC CHEMISTRY. 



