SIMPLE ORGAN^S. 73 



texture, resist decay for a mucli longer period. This firmer 

 part is called woody tissue, and consists of bundles of 

 cylindrical tubes or cells, of great length and toughness, 

 tapering at both ends. These are so minute that for a long 

 time they were confounded with fiber, but microscopic in- 

 vestigation has demonstrated that beyond a certain point 

 they cannot be divided, and that each of them is a perfect 

 and complete cylindrical tube, tapering at both ends, and 

 overlapping and adhering to each other. 



186. Here sclerogen is deposited, giving durability, 

 hardness, and elasticity to the plant or tree. Woody fiber 

 is as indispensable to trees as bones are to the bodies of 

 animals. 



187. In their early stage these woody fibers serve as 

 channels for the ascending sap; but as their cavities 

 become filled Avith solid matter, and are rendered dense 

 and firm, their developed uses unfit them for any secondary 

 work. They then only give strength and durability to the 

 plant. There is a third tissue, called vascular tissue, 

 which consists of hair-like tubes coiled spirally, and in- 

 closed in an external membrane. 



188. In their normal condition they are coiled up like a 

 corkscrew, but they may be drawn out to a great length. 

 If you take a young shoot of hop-vine or pokeweed, and 

 bend it down first on one side and then on the other, until 

 the outer skin is broken, and then draw the pieces gently 

 asunder, these spiral vessels may be plainly seen with the 

 naked eye. Such spiral vessels abound where great elas- 

 ticity is needed, as in the tender shoots of young plants, 

 the leaves and flowers, the petioles and peduncles. 



186. What is deposited here ? Its uses? Is woody fiber indispensable to 

 trees ? 



187. What do they serve in their earlier stages ? Does it continue this office ? 

 What is the third tissue called ? Its constitution ? Its appearance? 



188. What of its normal condition ? What is it capable of? Example ? Where 

 do spiral vessels abound ? 



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