46 OP THE SAP-VESSELS. 



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 di- 

 visible 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 car- 

 ried with force towards certain parts and in 

 certain directions, and that this can be accom- 

 plished by regular vessels only, not, asTourne- 

 fort supposed, by capillary attraction through 

 a simple spongy or cottony substance. 



I received the first hint of what I now be- 

 lieve 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, con- 

 firmed by experiments, and carried to much 

 greater perfection by Mr. Knight, whose pa- 

 pers in the Philosophical Transactions for 

 1801, 1804 and 1805 throw the most brilliant 

 light upon it, and, I think, establish no less 



