PROBLEM 2. The Fart Stems ami Roots Play hi Making Food 



M'5 



V\G. 1 80 There is variety in 

 water-condiicting tubes. A 

 and B are found in conifers 

 {they are called tracheids); 

 C is a tube foimd in flow- 

 ering plants. The walls are 

 thickened in different ways. 



O 



See Figure 181. If twigs are available 

 study one carefully by doing Exer- 

 cise 9. 



Along the sides of the stem are lateral 

 buds containing cells which can grow 

 into stems and leaves. In the spring, when 

 growth begins again some of the lateral 

 buds develop into branches. These stems, 

 like the main stem, lengthen near the tip, 

 form lateral buds, and before the end of 

 the season form a terminal bud. Thus 

 each year the branches extend farther 

 and farther out from the trunk and each 

 branch forms branches along its sides. 

 When a plant lives for many years it may 

 become very tall and wide-spreading. 

 We see that both stems and roots grow 

 in length only at the tips. 



How stems grow in width. In dicot trees 

 and shrubs the growing tips are notice- 

 ably thinner than other parts of the 

 stem. The thickest part of the tree trunk 

 is at the very bottom; the thickest part 

 of a branch is at its base. The fact that 

 the older parts of a stem are thicker than 

 the newer or younger parts is good evi- 

 dence that the stem of a tree or shrub is 



Terminal bud 



Lenticels 



Scars made by 

 scales of last 

 year's terminal 



bud 



Lateral bud 



Leaf scars 



The tubes through 

 which sap traveled 

 into the leaves 



Fig. 18 1 Tip of horse chestimt twig, about nat- 

 ural size. At what poi?7t woidd such a stem grow 

 in length? How crnt you tell how much this 

 stem increased in lejigth during one season? 

 Find the lateral buds. Find where leaves hai'e 

 been attached. Where do lateral buds arise with 

 relation to the leaves? 



