LECT. VII.] ANATOMY OF STEMS. 365 



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" ing to M. Mirbel, that the sap ascends." (Syst. 

 of phys. Bot. vol. ii. p. 119.) Now, Mirbel's 

 words are, " la seve monte par les gros vaisseaux." 

 (EUmens, t. i. p. 208.) The cells of the medul- 

 lary sheath are, indeed, oblong", and, therefore, 

 somewhat tubular, and to these probably Mr. 

 Knight refers ; but, admitting this, we cannot see 

 in what manner the ascent of the sap through 

 these cells can be aided by the spiral vessels, the 

 bundles of which are, in many instances, placed 

 at the distance of three or four diameters from 

 each other, and consequently the intermediate 

 cells are beyond the sphere of their influence. Mr. 

 Knight's opinions on this subject have certainly 

 much less weight than they usually and most de- 

 servedly possess. 



Grew, as I have already stated, first suggested 

 the idea of the spiral vessels acting as sap vessels, 

 and Du Hamel supposed he had detected them in 

 the performance of this function. Hedwig * also 

 asserts, that he observed the sap issuing from the 

 orifices of the spiral vessels in a horizontal section 

 of the stem of the Pumpkin, Cucurbita Pepo, and 

 the squirting Cucumber, Momordica Elaterium ; 

 and Senebier ^, more lately, remarked the same 

 occurrence in a section of the stem of the Sago 



* Fundamentum Hist. Nat. Muscorwn Jrondosoruntj Lip. 

 1732, p. 55. 



f Phys. veg. t. i. p. 107. 



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