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46 scarcely discernible. But philosophers sought in vain for any perforation, any thing like a tubular structure, in the woody fibres to countenance this hypothesis, for they are divisible almost without end, like the muscular fibre. This difficulty was overlooked, because of the necessity of believing the existence of sap-vessels somewhere; for it is evident that the nutrimental fluids of a plant must be carried with force towards certain parts and in certain directions, and that this can be accomplished by regular vessels only, not, as Tournefort supposed, by capillary attraction through a simple spongy or cottony substance.

I received the first hint of what I now believe to be the true sap-vessels from the 2d section of Dr. Darwin's Phytologia, where it is suggested that what have been taken for air-vessels are really absorbents destined to nourish the plant, or, in other words, sap-vessels. The same idea has been adopted, confirmed by experiments, and carried to much greater perfection by Mr. Knight, whose papers in the Philosophical Transactions for 1801, 1804 and 1805 throw the most brilliant light upon it, and, I think, establish no less