Page:An introduction to physiological and systematical botany (1st edition).djvu/79

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49
DR. DARWIN'S EXPERIMENTS.

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 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, 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