36 OF THE SAP-VESSELS. 



running longitudinally, often situated with great uni- 

 formity (an arguiuent for their great importance), and 

 found in all parts of a plant, although in some they are 

 so delicate as to be scarcely discernible. But Philoso- 

 phers sought in vain for any perforation, any thing 

 like a tubular structure in the woody fibres to coun- 

 tenance this hypothesis, they being divisible almost 

 without end, like the muscular fibre. This difficulty 

 was overlooked, because of the necessity of believing 

 the existence of sa[)-vessels somewhere ; for it is evi- 

 dent 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 regu- 

 lar vessels only, not, as Tournefort supposed, by ca- 

 pillary 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. Dar- 

 win's Phjjtologia, 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 expe- 

 riments, and carried to much greater perfection, b}^ 

 Mr. Knight, whose papers in the Philosophical Trans- 

 actions for 1801, 1804 and 1805 throw the most bril- 

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

 an entirely new theory of vegetation, by which th^ real 

 use and functions of the principal organs of plants are 

 r.ow for the first time satisfactorily explained. 



