dr. darwin's experiments. 49 



not exposed to the atmosphere. We know 

 that air acts upon the plant under ground, 

 because seeds will not vegetate in earth un- 

 der the exhausted receiver of an air-pump. 

 Phil. Trans. No. 25. I do not however 

 mean to contend that any of these spiral ves- 

 sels are air-vessels, nor do I see reason to 

 believe that plants have any system of longi- 

 tudinal air-vessels at all, though they must 

 be presumed to abound in such as are trans- 

 verse or horizontal. 



Dr. Darwin and Mr. Knight have, by the 

 most simple and satisfactory experiment, 

 proved these spiral vessels to be the channel 

 through which the sap is conveyed. 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 ex- 

 hibited a circle of red dots round the pith, 

 surrounded by an external circle of vessels 

 containing the white milky juice, or secreted 

 fluid, so remarkable in the fig-tree. Mr. 

 Knight, in a similar manner, inserted the 



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