34 



GROWTH AND WORK OP PLANTS 



provided with a special sheath of spongy tissue ( the velameri) 

 which absorbs moisture from the air. Another example common 

 in greenhouses is the wandering Jew (Tradescantia). The 

 aerial roots grow from the joints, usually two roots from each 

 joint. Good examples of aerial roots can be seen in the case 

 of the climbing poison ivy, English ivy, trumpet creeper, etc. 

 These serve to hold the vine to the tree or other support on which 

 it is climbing. 



56. Bracing roots, or prop roots. In some plants where 

 the fibrous-root system in the soil is not sufficient to support the 



heavy plant upright, aerial 

 roots are developed a short 

 distance above the ground and 

 as they reach the soil serve to 

 prop or brace the plant. Good 

 examples are seen in the Indian 

 corn, and in the screw pine 

 grown so commonly in green- 

 houses. A classic example of 

 prop roots is seen in the banyan 

 tree of India where numerous 

 roots grow downward from the 

 wide spreading branches. The 

 mangrove along the coast in 

 the subtropical regions of Florida is another example. 



57. Strangling roots. In some tropical countries there are 

 trees (Clusia) which begin their life as seedlings on other trees 

 from seeds which have lodged in the fork of a branch or some 

 other landing place. Slender roots grow down to the ground, one 

 of which forms the lower part of the trunk of Clusia. Other roots 

 coil around the foster tree. When the Clusia becomes a large 

 tree these roots wrapped around the foster tree tightly strangle 

 and kill it.* 



Fleshy roots, or root tubers. These are roots or por- 



Fig. 37- 

 Bracing roots of Indian corn. 



* See "A Tragedy of the Forest," Torreya 8, 253-259, 1908. 



