OF THE S\P-VESSF,LS, SJ. 



fibre. This difficulty was o^ erlooked, because of the 

 necessity of believing the existence of sap- vessels some- 

 where ; 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 accom- 

 plished by regular vessels only, not, as Tournefort sup- 

 posed, 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, established no less than an entirely new 

 theory of vegetation, by which the real use and func- 

 tions of the principal organs of plants are now for the 

 first time satisfactorily explained. 



In a \oung branch of a tree or shrub, or in the stem 

 of an herbaceous plant, are found, ranged round the 

 centre or pith, a number of longitudinal tubes or vessels, 

 of a much more firm texture than the adjacent parts, 

 and when examined minutely, these vessels often appear 

 to be constructed with a spiral coat. This may be seen 

 in the young twigs and leaf stalks of Elder, Syringa, 

 and many other shrubs, as well as in numerous herba- 

 ceous plants, as the Peony, and more especially many 

 of the Lily tribe. If a branch or stalk of any of these 



