38 DR. DARWIN'S EXPERIMENTS. 



philosopherspresumed, with certain vessels of the leaves 

 and flowers, of an oval or globular form, but destitute 

 of a spiral coat. These Jatter do really contain air, 

 but it rather appears from experiment that they have 

 no direct communication with the former. Thus the 

 tubes in question have always been called air-vessels, 

 till Darwin suggested their real nature and use*. He 

 is perhaps too decisive when he asserts that none of 

 them are air-vessels because they exist in the root, 

 which is not exposed to the atmosphere. We know 

 that air acts upon the plant under ground, because 

 seeds will not vegetate in earth under the exhausted 

 receiver of an air-pump. Phil. Trans. 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 system of longitudinal air-vessels 

 at all, though they must be presumed to abound in such 

 as are transverse 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 com- 

 mon Fig-tree about an inch deep in a decoction of 

 madder, and others in one of log-wood. 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 



* Du Hamcl, indeed, once suspected that they contained " highly 

 rarened sap," but did not pursue the idea. 



