PIN US SYL VESTRIS. 147 



/. r ^\\t scale leaves. 

 Examine with a high power. Study 



17. The pith cells. Note 



a. The shape and arrangement. 



1 8. The xylem. Note 



a. Near the pith parenchyma a cluster of spiral and 

 reticulated vessels. Notice the irregularity and 

 closeness of the spiral thickening. 



b. The tracheides, making the bulk of the xylem. 

 Note 



i. Their shape. Observe their ends. 



ii. Their thickened walls. 



iii. Their markings, bordered pits. In the young- 

 est part of the xylem study the structure of one 

 of these pits. Observe the two concentric 

 circles they present. Note which is more dis- 

 tinct. Compare with transverse section and 

 discover the cause of this appearance. The 

 outer circle is at the point where, in section 

 (see diagram at A. 9. c. iv.) the arms of the 

 Y diverge from its stem ; the inner is the 

 edge of the opening in the bowed walls. By 

 examining this section thoroughly, chance 

 sections of the pits may be found which will 

 further elucidate their structure. 



iv. The size of the pits compared with the breadth 

 of the fibers, and their arrangement on the 

 fibers. 



v. The large thin spots on the walls of the cells 

 of the medullary rays, where they join the 

 adjacent tissues. 



c. Between the tracheides and the spiral vessels a 

 few intermediate cells with plain pits nearly or 



