LESSON 10.] POEOUS AND DOTTED DUCTS. 



Coniferous woods have been easily arranged in species ; a figure of 

 fossil wood, from a cone-bearing tree, is presented (Fig. 58). In this 

 illustration the cell is filled with four rows of alternate ducts. The 

 small spot in the centre of each concave disc, is supposed to be all 

 that remains of a series of vessels formerly (during the development 

 of the plant) existing in that situation. 



FIG. 59. 



FIG. 58. 



Single duct, from Fossil wood. 

 (Much magnified.) 



194. The vessels constituting the porous, dotted, or 

 pitted vascular tissue, are continuous tubes of large 

 size, and usually present broad or oblique extremities ; 

 but sometimes the terminations are pointed (Fig. 59). 

 Their dotted, or pitted, appearance is supposed to de- 

 pend on the mode in which the encrusting matter is 

 formed inside. In place of being deposited equally 

 over the whole surface of the membrane, as in ordinary 

 woody fibre, it leaves rounded uncovered spots at vari- 

 ous intervals, and these, when viewed by transmitted tlssue - 

 light, appear from their thinness to be perforations or holes. 



195. In old dotted ducts it frequently happens, that the thin 

 membrane of the dots, or pits, has become absorbed, 



and actual perforations taken place. 



196. Dotted vessels often present a jointed Appear- 

 ance, such as is shown in Fig. 60 ; in such cases, they are 

 seen to be formed of dotted cells placed end to end, so 

 as to form continuous cylindrical tubes. 



197. Dotted ducts are abundantly found in the wood 

 of trees, and they constitute the large rounded openings 

 seen in transverse sections of the stems of oak, pop- 

 lar, willow, &c. They also abound in the Bamboo, and 

 in other plants of rapid growth. 



r r & Dotted vessels. 



198. Porous ducts are very numerous in the Locust 



tree, and are of great beauty (Fig. 61) ; it will be seen that the lower 

 3 



Porous 



FIG. 60. 



