4i6 



RELATION TO KNVIRONMKN'T. 



tories. The amount of moisture is so threat in tliese tropical 

 regions that the roots are abundantly supplied without the soil 

 relation. Certain of the roots hant; free in the air and are pro- 

 vided with a special sheath of sponj^v tissue called the velamen, 

 through which moisture is aljsorbed from the air. Other roots 

 attach themselves to the trunk or branches of the tree on which 

 the orchid is growing, and furnish the support to the epipliyte, 

 as such plants are often called. Among the tangle of these 

 cUnging roots falling leaves are caught. Here they decay and 

 nourishing roots grow from the clinging roots into this mass of 

 decaying leaves and supply some of the plant fooci. Aerial 

 roots sometimes possess chlorop)h\'ll. 



There are a number of plants, however, in temperate regions 

 which ha\'e aerial roots. These are chiefly used to gi\'e the stem 

 support as it climbs on trees or on walls. They are s<mietimes 

 called clinging roots. A common example is the climbing poison 

 ivy (Rhus raflicans), the trumpet creeper, etc. Such aerial roots 

 are called adventitious roots. 



797. Bracing roots, or prop roots. — These are developed in a 

 great variety of plants and serve to brace or prop the jilant where 



the fibrous-root svstem is in- 

 sufficient to support the heavy 

 .shoot system, or the shoot sys- 

 tem branches so \\-idely props 

 are needed to hold up the 

 branches. In the common In- 

 dian corn several whorls of 

 bracing roots arise from the 

 nodes near the ground and ex- 

 tend outward and downward to 

 the ground, though the upper 

 whorls do not always succeed in 

 reaching the ground. The 

 screw-pine so common in 

 greenhouses affords an excellent example of prop roots. The 

 roots are cjuite large, and long before the root reaches the soil the 



Fig. 44S. 

 Bracing runts of Indian < 



