LECT. VIJ.] ANATOMY OF STEMS. 361 



may be the arrangement, it appears to be in a 

 great degree regulated by the disposition of the 

 leaves, into which the spiral vessels in every in- 

 stance direct their course, leaving for that pur- 

 pose the medullary sheath, and traversing the 

 wood, a little below the insertion of each leaf. 



As the medullary sheath forms the only parti- 

 tion between the bark and the pith in the tender 

 succulent shoot, before the ligneous matter is de- 

 posited, and is in its texture lax, and incapable of 

 affording sufficient support to the delicate coats 

 of vessels, such as are found in the alburnum, if 

 these were distended with ascending sap, the 

 vessels that run through it are of a different 

 structure from those of any other part of the ve- 

 getable. The elastic thread of which these spiral 

 vessels are formed is tough, and possesses irrita- 

 bility ; and being stimulated to action by the effort 

 of the sap to dilate the diameter of the vessel, 

 contracts in its length in each coil alternately, and 

 after each contraction again returns to its first 

 state, producing a vermicular motion, which 

 enables these vessels to conduct forward the sap. 

 Thus : the contraction in length of the portion of 

 the thread which forms the first coi}, lessens the 

 diameter of that portion of the tube, and hence the 

 fluid contained within in it will be displaced and 

 moved either upwards or downwards ; but as the 

 resistance opposed to its return, or movement 



