

SECT. II. TUBES. 351 



Vine. They have generally been characterized 

 under the denominations of proper vessels, lympha- 

 tics, and tracheae. But as this is rather premature 

 reference to their different uses, which is besides 

 not altogether correct, I shall adopt with a little al- 

 teration the denominations introduced by M. Mir- 

 bel, as arising from their form or structure. The 

 first and primary division founded upon this princi- 

 ple is that by which they are distributed into large 

 tubes, and small tubes.* 



-SUBSECTION I. 



Large Tubes. The Large Tubes are tubes dis- 

 tinguishable by the superior width of the diameter 

 which they present on the horizontal section of the 

 several parts of the plant. 



In herbaceous plants they are represented by Their 

 M. Mirbel as being always found in the centre of species, 

 the longitudinal fibres ; while in woody plants they 

 are often dispersed at random ; though they some- 

 times form regular groups, which are sometimes 

 concentric circles constituting the principal mass 

 of the ligneous layers. They are generally to be 

 found in great abundance surrounding the medul- 

 lary canal. They are found also in the bark, and 

 are capable of being traced from their origin in the 

 extreme fibres of the root, to their termination in 

 the extreme summit of the plant, uniting in the 

 body of the root, traversing the collar, penetrating 

 * Phy*. Veget. vol. i. p. 6l. 



