38 VEGETABLE ORGANOGRAPHY. 



of the pith ; in Monocotyledons they occur in each of 

 the woody bundles ; ..they are very abundant in the 

 centre of the stem of the Lycopodiaceae. 



The largest annular vessels known are those in the 

 stem of the Balsam. 



Kieser considers these vessels to be composed of 

 parallel rings, wliich, according to him, are of an analo- 

 gous nature to the tissue of the trachege, and are capable, 

 in certain cases, of gradually changing into spires (*PL 

 2, fig. 7 b,) : these rings are sometimes, according to 

 him, very slightly adherent to the membranous tubes 

 formed by the walls of the neighbouring cellules. 



Mirbel considers them to be tubes marked with 

 parallel slits ; others, as tubes provided with opaque 

 parallel lines, which are of an analogous nature to the 

 dots of dotted vessels. 



Perhaps under the name of Annular Vessels (vais- 

 seaux rayes ou annulaires ) we really confound two 

 different structures. 



Section IV. 



Of Dotted Vessels. 



I include, with Tre\iranus, under the name of 

 Dotted Vessels, (vaisseaux ponctues, vasa punctata) 

 those which Mirbel calls Porous Tubes or Vessels, 

 (vaisseaux ou tubes poreux,) and Kieser, Spiral Dotted 

 Vessels, (vaisseaux spiraux ponctues,) PL 2, fig. 2 b, b, 

 5 b. Their ordinary state, under the microscope, is 

 the form of a cylindrical tube, the walls of which 

 present transverse series of opaque dots : they differ, 



* From an excellent plate iii Dr. Lindley's valuable " Introduction to 

 Botany." 8\o. London, 1836. 



