PROSENCHYMATIC WOOD-ELEMENTS. 5 



III. CORK-CELLS. 



For the examination of these cells, the student should begin 

 with the soft and close-textured "velvet" cork procurable at 

 any apothecary shop. Let the sections be made in at least two 

 directions at right angles to each other, and if possible let them 

 pass through one of the lines of demarcation of the cork : note 

 any differences of shape and size presented by the cells at that 

 place. 



The young stems of any of our common currants give in cross- 

 section excellent illustrations of cork-cells (see pages 74-76) 

 and of their development. Test these and similar specimens of 

 cork-cells for the presence of cutin or suberin (see 26, 54, 161). 



IV. PROSENCHYMATIC WOOD-ELEMENTS. 



These elements (see pages 78-87) can be studied to best ad- 

 vantage after very careful maceration, as directed in 70. Long 

 wood-cells, woody fibres, and tracheae (or ducts), are easily 

 separable from each other by such chemical means, and are 

 general!}' identified with facility. Abundant material for the 

 demonstration of tracheae is afforded by the fibro- vascular bundles 

 (198) of herbs and by the ligneous parts of our common trees 

 other than the Coniferce. There appears to be no special need 

 of specifying the ligneous plants which can be most successfully 

 emploved for demonstrations of the woody elements. Magnolias, 

 Tulip-tree, woody Leguminosae and Rosaceae, Urticaceae, and 

 Cupuliferae are all satisfactory as sources of material. 



Good examples of tracheids are procurable from species of 

 Coniferae, such as Pines, Firs, Spruces, etc. These should be 

 examined in all stages of development and from all points of 

 view, particular attention being directed to the marked difference 

 between the radial and tangential aspects of the cells. 



Cells which have been separated from each other mechanically 

 and have not been previously acted on by chemicals should be 

 studied with reference to their behavior under the action of 

 iodine and other reagents, it being possible to demonstrate the 

 existence of thin layers or "plates" which compose the wall. 

 Iodine colors the fresh cells yellow ; investigation shows, how- 

 ever, that the inner wall or plate of the cell is not much, if at 

 all, colored by the reagent, the color being confined to an outer 

 and a middle wall or plate. When the cells thus treated with 

 iodine are touched with concentrated sulphuric acid, the outer 

 and middle plates remain yellow, while the inner plate turns 



