PIN US SYLVESTRIS. 153 



8. The mesophyll. Note 



a. The shape of the cells, and the number of rou>s 

 between the hypoderma and bundle sheath. 



b. The infolJings of the wall, dividing the cavity 

 into recesses. Observe the position of the most 

 prominent of these infoldings in the outermost 

 row of mesophyll cells. Observe occasionally 

 (usually near a stoma) branched cells. Deter- 

 mine the relation of these to the cells with simple 

 infoldings. 



c. In fresh specimens, the abundant chlorophyll. 



d. The resin ducts ; compare their structure with 

 those of the stem. Notice the thick walls of the 

 cells of the sheath. 



9. Draw a few mesophyll cells showing also a resin duct. 

 10. The fibro-vascular region. Study 



a. The bundle sheath ; shape and contents of the 

 cells. 



b. The two masses of small cells, the fibro-vascular 

 bundles, somewhat separated from each other and 

 obliquely placed. Note the well-marked division 

 into two areas : 



i. The xylem, next the flat side of the leaf, 

 consisting of spiral and reticulated ves- 

 sels and tracheides, arranged in radial rows. 



ii. The /&!, next the convex side of the leaf, 

 consisting chiefly of undeveloped sieve cells. 



iii. The radial rows of parenchyma (like medul- 

 lary rays), passing through both xylem and 

 phloem. 



iv. In the xylem area, occasionally a poorly 

 developed resin duct. 



