VEGETABLE ORGANIZATION. 



65 



the names of moniliform, Jointed or beaded vessels.* 

 Traces of the membranous partitions sometimes remain where 

 their obliteration has been only partial, leaving transverse 

 fibres. The conical terminations occasionally observable 

 in the vessels of plants also indicate their cellular origin, t 



14 15 



1 



The membrane constituting the tube is sometimes simple, 

 like those of the simple cells: but it frequently contains 

 fibres, or other internal coatings, corresponding to those met 

 with in the more compound cells. The vessels in which the 

 internal fibres run in a spiral direction (Fig. 14,) are deno- 

 minated trachese, or spiral vessels; or, from their being found 

 very constantly to contain air, they are often called air tubes. 

 Their diameter is generally between the 1000th and the 

 300th part of an inch. These spiral, or air vessels, pervade 

 extensively the vegetable system. The threads they con- 

 tain are frequently double, treble, quadruple, or even still 

 more numerous: they are of great length, and when the ex- 

 ternal membrane of the vessel is divided, they may easily 

 be drawn out and uncoiled, their elasticity enabling them to 

 retain their spiral shape. The object of this structure ap- 

 pears to be that of keeping the cavity of the tube always 

 pervious, by presenting resistance to any external force tend- 

 ing to compress and close itij: 



* Mirbel g'avc tlicm the name of" Vaisseaux en chapelet." 



f This theory of the derivation of vessels from cells was first advanced by 

 Treviranus. 



+ Vessels are sometimes met with which appear to be formed simply by 

 the coils of a spiral fibre in close juxtaposition, and unattached to any ex- 

 ternal envelope, or connecting" membrane. 



Vol. I. 9 



