45 



win placed a plant of spurge (Euphorbia Hclmcofiia] 

 in a decoction of- madder, and in a few days the co- 

 loured fluid passed along the middle rib of the leaf, 

 and ran by several branches to the extremity of it 

 upon the upper surface ; while, on the under side, 

 a system of vessels was seen coming from the end 

 ef the leaf, and conveying a milky fluid, which de- 

 scended finally into the foot stalk* : and Mr Knight 

 has traced the vessels conveying this peculiar juice 

 from the leaves into the cortical layers of the inner 

 bark f Now the difference in the appearance and 

 qualities of this returning fluid was probably effect- 

 ed in the leaves by the agency of the air, and in this 

 way the succus firo-firius of plants seems to be pre- 

 pared. But can we suppose, either the air or its 

 oxygenous portion to be received into these minute 

 vessels, already filled with fluids, to be there decom- 

 posed, then transformed into, and at length emitted 

 as carbonic acid ? If so, by what passages can it en- 

 ter, and how afterwards can it be expelled ? It can- 

 not enter by the transpiring or exhaling pores, be- 

 cause the fluids constantly escaping from them must 

 obstruct its admission ; nor have we any instance of 

 any organ performing two contrary actions at one 

 and the same time, a condition which this supposi- 

 tion would involve. If, again, it enter by peculiar 

 vessels fitted for the absorption of air, (whose exist- 

 ence however is not proved), by what vessels can it 

 afterwards be emitted as carbonic acid ?. Not surely 



* Phytologia, p. 43. 



f Philosophical Transactions, 1801, p. 337. 



