32 



VEGETABLE PHYSIOLOGY. 



[LESSON 10. 



LESSON X. 



POROUS AND DOTTED DUCTS. 



188. In addition to the spiral, and other vessels already de- 

 scribed, tubes or canals are also found in plants ; these are called 

 duds, and give to woods their various degrees of porosity. 



189. Spiral vessels can be easily unrolled ducts possess no such 

 capability, and hence the word duct is limited to such vessels as can- 

 not be unwound. 



FIG. 54. 190. Dotted ducts are peculiar to firs, pines, and cone- 

 bearing trees; they consist of spindle-shaped, or fusiform 

 cells (Fig. 54), bearing a variable number, according to spe- 

 cies, of saucer-shaped discs, each having a small circle in the 

 centre. 



191. These peculiar markings are formed by concave de- 

 pressions on the outside of the walls of contiguous tubes, 

 which are closely applied to each other, forming lenticular 

 (shaped like a lens) cavities between the vessels, like two 

 watch-glasses in close apposition ; when seen by transmitted 

 light they appear like discs. 



192. This structure is common to, and characteristic of 

 duct"f the * ne cone-bearing trees ; the number of discs, or ducts, in a 



Fir * given cell is found to differ in the several species, thus the 

 pine (Fig. 55) has only a single row in each cell ; the pinus strobus 

 (Weymouth pine, Fig. 56) has two rows of discs, arranged on the 

 same plane; whilst Araucaria has two and three rows (Fig. 57), ar- 

 ranged alternately. 



FIG. 55. 



FIG. 56. 



FIG. 57 



Dotted ducts of 

 the Pine. 



Ducts of Pinus 

 Strobus. 



Ducts, Araucaria. 



193. These characters are so constant that, by the number of 

 discs in a cell, together with the mode of their arrangement, Fossil 



