OF THE STRUCTURE OF WOOD AND HERBS. 

 Fig. 138. 



91 



A longitudinal section of the same tree is delineated in figure 138, as it ap- 

 pears when magnified likewise four hundred times. The structure is exquisitely 

 beautiful : the straight lines a a, b 6, c c, d d, &c., are sections of the sides of 

 the vessels or tubes which run lengthwise of the trunk, the ends of which are 

 disclosed in the woven lines of the last figure. The sides of the vessels are seen 

 studded with rows of small circular disks which have received the name of 

 glands ; each disk having a small circular ring around its central point. The form 

 of the disk is not always the same ; it is generally circular but frequently oval ; 

 and when closely arranged together they assume an angular shape. In some 

 species of Pines the disks run through the vessels in singTe rows ; but in others, 

 as in the case of the White Pine, they occur, as is obvious, both in single and 

 double rows. It is a remarkable fact, that throughout the entire genus of the 

 living, true Pines, no more than two rows of disks are ever found in the longitu- 

 dinal section of a single vessel, and that when a double row occurs, the corres- 

 ponding disks of each row are placed side by side. The vessels are sometimes 

 found without disks. 



A class of cone-bearing trees, allied to the Pines, is known by the name of 

 Araucaria. It includes some of the loftiest living trees, and the well-known 

 species that grows in Norfolk Island, near New South Wales; and which bears 

 the name of the Norfolk Island Pine. This class possesses certain peculiarities 

 of structure which are at once detected by the microscope, and distinguish it 

 from the true Pines. 



In figure 139 a longitudinal section of the Norfolk Island Pine is displayed, 

 Fig. 139. magnified to the same extent as the two preced- 



ing figures. The disks that cover the sides of the 

 vessels are here arranged in double and triple 

 rows ; and in the Araucarias the rows belonging 

 to the section of a single vessel vary in number 

 from one to four. 



Another peculiarity is also perceived in the 

 shape of the disks, which, instead of being gen- 

 erally circular like those of the pine, are for the 

 most part bounded by straight lines. The disks 

 also of both the rows in a double row, are not 



