150 BOTANY: PRINCIPLES AND PROBLEMS 



reproductive structures. By skillful manipulation, the growth 

 and differentiation of the entire plant may thus to a certain degree 

 be brought under control. 



QUESTIONS FOR THOUGHT AND DISCUSSION 



441. What is the chief difference in method of growth between 

 animals and plants? 



442. With what other important difference between animals and 

 plants is this difference in method of growth associated? 



443. What disadvantage would there tend to be in very large cells? 

 In very small ones? 



444. What factors are there which tend to limit the size to which 

 a tree can grow? 



445. In what direction, or plane, with reference to the rest of the 

 stem are most of the cell divisions which take place at the terminal grow- 

 ing-point of the stem? at the cambium? 



446. What difference in shape would you expect to exist between the 

 cells at a terminal growing-point and those at a cambium? 



447. The zone in which elongation occurs in the root tip is much 

 shorter than it is in the stem tip. Of what advantage is this fact to the 

 plant? 



448. Growing-points of plants are usually good to eat and in a few 

 cases are important human foods. Why is this so? 



449. The bark will separate very easily from the wood of a twig in 

 the spring but usually at no other season. Explain. 



450. If a nail is driven into a tree trunk at a point 3 feet from the 

 ground, what position will this nail occupy in the tree 30 years later? 

 What evidence from observation have you for your answer? 



451. In just what part of the stem does growth of the pith take place? 

 of the wood? of the cortex? of the bast? of the epidermis? 



452. Is the pith in a one-year-old twig wider or narrower than it is 

 in a 20-year-old branch grown from that twig? 



453. Where would you find the cortex in a tree trunk? 



454. What important changes in the size and character of its tissues 

 take place as a one-year-old twig grows into a 20-year-old branch? 



455. Why is the bark of a tree almost always rough and cracked? 



