DR. DARWIN'S EXPERIMENTS, So 



vegetate in earth under the exhausted receiver of an air- 

 pump. Fhil. Traits. No. 23. I do not however mean 

 to contend that any of these spiral vessels are air-vessels, 

 nor do I see reason to believe that plants have any sys- 

 tem of longitudinal air-vessels at all, though they must 

 be presumed to abound in such as are transverse or hor- 

 izontal. 



Dr. Darwin and Mr. Knight have, by the most sim- 

 ple and satisfactory experiment, proved these spiral 

 vessels to be the channel through which the sap is con- 

 veyed. The former placed leafy twigs of a common 

 Fig-tree about an inch deep in a decoction of madder, 

 and others in one of logwood. After some hours, on 

 cutting the branches across, the coloured liquors were 

 found to have ascended into each branch by these vessels, 

 which exhibited a circle of red dots round the pith, sur- 

 rounded by an external circle of vessels containing the 

 while milky juice, or secreted fluid, so remarkable in 

 the fig-tree. Mr. Knight, in a similar manner, inserted 

 the lower ends of some cuttings of the Apple-tree and 

 Horse-chesnut into an infusion of the skins of a very 

 black grape in water, an excellent liquor for the pur- 

 pose. The result was similar. But Mr. Knight pur- 

 sued his observations much further than Dr. Darwin 

 had done ; for he traced the coloured liquid even into 

 the leaves, " but it had neither coloured the bark nor 

 the sap between it and the wood ; and the medulla was 

 not affected, or at most was very slightly tinged at its 

 edges." Phil Trans, for 1801, />. 335. 



The result of all Mr. Knight's experiments and re- 

 marks seems to be, that the fluids destined to nourish a 



