68 STRUCTURE OF HIGHER PLANTS 



V. Observe the side view of the cells in the lower epidermis. Describe the 

 appearance of the epidermal cells, guard cells, and stomata. 



VI. Discuss the importance of the translucent character of the epidermal 

 cells. 



VII. What speciahzation of structure for function is shown in a green leaf? 

 Give at least five examples. 



VIII. Secure two leaves from a rubber plant. Coat the upper epidermis of 

 one and the lower epidermis of the other with vaseline. Seal the ends of the 

 stems with vaseline. Keep them in a cool place for several days. 



A. State your observations. 



B. Suggest a second use for the stomata. 



Problem. Study of the vascular and supporting tissues in a leaf. 



Break off a narcissus leaf and separate some tissues from the broken end. 

 Mount on a glass slide. 



I. Observe some of the cells with spiral thickenings on the walls. These 

 are tracheids (xylem). In order to keep open the passageway of the tubes 

 formed by these cells, spiral thickenings of wood form on the wall. Originally, 

 there were cross walls where one cell ended and another began, but the parti- 

 tions have been absorbed, leaving an uninterrupted canal. Water travels 

 up through these tracheids. 



II. Thare are other cells with sievelike plates at regular intervals. These 

 are thp sieve tubes (phloem). Through these structures materials pass down 

 the plant from the leaves to the roots. 



A. Trace a material that travels from leaves to roots. 



B. Why do materials tend to pass more quickly down than up? 



C. Wliat is a possible advantage of the sievelike cross walls in the 

 sieve tubes? Each sieve tube is a single row of elongated cells placed 

 end to end and the thickened end walls of these cells have numerous 

 perforations. 



III. There are also supporting tissues called wood fibers that have very thick 

 walls of cellulose. Some of them consist entirely of cellulose and compose the 

 supporting structure in a plant. 



IV. Place the roots of a complete narcissus plant or any other young plant 

 in red ink or eosin in order to show the continuous pathway from roots to leaves. 



A. What evidence of the passage of fluids do you observe ? 



B. Trace the liquid that travels up in a plant from the point of en- 

 trance to the cells that use it. 



V. Draw and label as many of the tissues as you have observed. 



