48 LABORATORY AND FIELD EXERCISES 



border. Determine accurately the structure of 

 these cambium cells. Note the orderly arrange- 

 ment of the tissues of the xylem and phloem 

 which arise from the cambium. Why is this so ? 

 Do the wood rays arise similarly from the cam- 

 bium ? Are the rays continuous from the time of 

 their origin to old age in a tree? Be able to 

 explain this point. 



d. The wood. Study carefully the following cells and 

 tissues of the wood : 



(1) Water ducts. Their size, contents, and cell-wall 



thickenings. Why are they so much larger than 

 the surrounding cells ? 



(2) Wood fibers. Are they thick-walled or thin- walled ? 



Have they cell contents ? What is their function 

 in a living tree trunk ? Are they structurally 

 adapted to their function ? 



(3) Annual rings. Study the relation of ducts and 



fibers at the junction of two annual rings. Is the 

 boundary line between spring and summer wood 

 a sharp one ? Why is the annual ring so plainly 

 visible in stumps or sawn logs ? 



(4) Wood rays. Observe their cell structure carefully 



and compare it with that of the surrounding dead 

 ducts and fibers of the wood. Do you find pores 

 in the end walls of these cells and protoplasmic 

 continuity through these pores ? This can be seen 

 only in very thin and well-stained preparations. 

 What is the function of wood rays ? 



(5) Wood parenchyma. Observe the amount and dis- 



tribution of living cells with darkly stained con- 

 tents in the wood outside of the rays. Are these 

 cells in isolated or in connected masses ? Are 



