ELEMENTARY ORGANS. 5 



26. VASCULAR TISSUE (Trachmchyma) consists of very thin- 

 sided cylinders tapering to each end, and having a spiral fibre 

 generated in their inside. 



27. Of this kind of tissue spiral vessels 1516 are the type. 

 Their fibre is of a highly elastic nature, and is capable of un- 

 rolling when stretched. 



28. Spiral vessels are found in the medullary sheath, and in 

 all parts that emanate from it, especially the veins of the 

 leaves, and everything that is a modification of them. 



29. They are usually absent from the wood and bark. 

 They, however, occur in these and other unusual parts in a 

 few extremely rare cases ; as in the wood, and bark, and pith 

 of Nepenthes. 



30. The spiral vessels appear intended for the conveyance 

 of air, which has been found to contain 7 or 8 per cent, more 

 oxygen than the atmosphere. 



31. Ducts are transparent tubes, the sides of which are 

 marked with rings, bars, or transverse streaks. 



32. They are slight modifications of the spiral vessel, differ- 

 ing principally in being incapable of unrolling ; and, in some 

 cases, in the turns of the spiral fibre being distant or broken, 

 or even, in appearance, branched. 



33. In those cases where the turns of the spire actually 

 touch each other, the ducts, which are then called closed, can 

 only be distinguished from spiral vessels by their inability to 

 unrol ; while at rest they appear to be absolutely the same. 



