Further Growth and Duration of Plants 29 



of roots some of which serve to fix the plant, while others stand 

 out toward the light. They are green and assist the leaves in 



FIG. 34. White Potato. 



FIG. 35. The brae 

 ing roots of Indian 

 Corn (Zea mays). 



doing their work, while their outer dead cells soak up moisture 

 like a sponge. 



Roots borne in the air are called aerial roots. The ivy 

 and climbing cactus use their aerial roots in climbing. They 

 have underground roots which absorb food material. 



THE WAY TREES GROW. 



Some plants continue growth, both above and below the 

 surface of the soil, year after year. If such a plant grows to a 

 height of 20 feet or more, and the lower branches remain 

 undeveloped and fall off, so as to leave a central stem or trunk 

 exposed, it is a tree. If it forks continuously from the base 

 and has no strong central trunk, it is called a shrub. 



Roots reach out over a large area or they may extend down 

 to a great depth, so that trees having the two kinds of root 

 systems may be planted close together. Should Oaks and Blue 

 Gums be planted side by side ? Shade trees around a garden 

 should have deep growing root systems, so that they will not 

 remove the surface moisture required for the flowers or vege- 

 tables. 



Plants in the germinator showed root-hairs near the tip. 

 They died away further back where the side roots were forming. 



