404 



VEGETABLE TISSUES 



-DUCTS. 



Rhubarb. Not unfrequently, however, we find all forms of Ducts 

 in the same bundle, as seen in Fig. 211. — The size of these Ducts 

 is occasionally so great as to enable their openings to be distin- 

 guished by the unaided eye : they are usually largest in Stems 

 whose size is small in proportion to the surface of Leaves which 

 they support, such as the common Cane, or the Vine ; and, gene- 

 rally speaking, they are larger in woods of dense texture, such as 

 Oak or Mahogany, than in those of which the fibres, remaining 



Fig. 211 



Longitudinal section of stem of Italian Reed : — a, Cells of the 

 Pith ; b, Fibro-Vascular bundle, containing 1, Annular Duct ; 

 2, Spiral Duct ; 3, Dotted Duct, with Woody Fibre : c, Cells of 

 the integument. 



unconsolidated, can serve for the conveyance of fluid. They are 

 entirely absent in the Coniferce. 



297. The Vegetable tissues whose principal forms have been 

 now described, but among which an immense variety of detail is 

 found, may be either studied as they present themselves in thin 

 Sections of the various parts of the plant under examination, 

 or in the isolated conditions in which they are obtained by 



