296 STRUCTURE OF STEMS AND ROOTS 



does not occur in growing points (buds) of stems. In their 

 internal structure roots differ from stems, especially when 

 very young. In older roots the differentiation is not so 

 marked. (Fig. 467.) Young roots have the radial arrange- 

 ment of phloem and xylem (490). The number of xylem 

 strands radiating from the center differs with the plant. 

 In roots also there is almost uniformly a true endodermis. 

 This layer is just within the cortex and is composed of 

 rather thick-walled cells. However, many rhizomes and 

 stems have a true endodermis. 



501. To study pea roots: From the roots of the pea 

 a few weeks old cut thin cross-sections; stain and mount. 

 With the aid of the low power make a sketch showing the 

 arrangement of the str&nds of wood and bast, and also the 

 amount of fundamental tissue. Use the highest power and 

 draw a portion including one strand of wood and two of 

 bast. In this part, draw the tissues from the center out 

 beyond the endodermis. Sections may also be made of the 

 roots of germinating pumpkins or squashes. 



Revjew. — Name two types of stems occurring in flowering plants. 

 Describe each and give examples to illustrate them. Give the plan 

 of arrangement of bundles in fern stems. How many types of bundles 

 are there? Upon what do their differences depend? Describe and give 

 examples of collateral bundles. What difference is there between open 

 and closed collateral bundles? Give examples of each. Describe 

 and give examples of concentric bundles. Radial arrangement. What 

 is secondary thickening? What plants show it? What is the layer 

 called that forms the new cells in a bundle? When is this layer most 

 active? Describe the work of this layer. What part of each bundle of 

 a dicotyledon is found in the bark? What are lenticels? What is phel- 

 logen? Describe the work of phellogen in any plant you have studied. 

 Whore is the root cap? What is its use? Describe fully the structure 

 of roots, telling how they differ from stems. 



