ANATOMY OF STEMS. 207 



except by the spiral vessels, which have not yet been 

 discovered in any layer of formed wood subsequent 

 to the first ; for their apparent existence in stems and 

 branches of several years' growth is owing to the 

 lignilication of the Medullary Sheath. The cells 

 which are between the layer of spiral vessels and the 

 pith, and which are the site of the coloring matter, 

 when this part of the stem is green, have a cribriform 

 structure. The variable arrangement which these 

 spiral vessels present in different plants, appears to be 

 in a great degree regulated by the disposition of the 

 leaves, into which the spiral vessels in every instance 

 direct their course, leaving for that purpose the Med- 

 ullary Sheath, and traversing the wood, a little below 

 the insertion of each leaf. 



As the Medullary Sheath forms the only partition 

 between the bark and the pith in the tender succu- 

 lent shoots, before the ligneous matter is deposited, 

 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 distend- 

 ed with ascending sap, the vessels that run through it 

 are of a different structure from those of any other 

 part of the vegetable. The elastic thread of which 

 these spiral vessels are formed is tough, and possesses 

 irritability ; and being stimulated to action by the ef- 

 fort of the sap to dilate the diameter of the vessel, 

 contracts in its length in each coil alternately, and af- 

 ter each contraction again returns to its first state, 

 producing a vermicular motion, which enables these 

 vessels to conduct forward the sap. Thus : the con- 

 traction in length of the portion of the thread which 

 forms the first coil, lessens the diameter of that por- 

 tion of the tube, and hence the fluid contained within 

 it will be displaced and moved either upwards or 

 downwards ; but as the resistance opposed to its re- 



