VEGETABLE ORGANIZATION. 



73 



observable in the vessels of plants also indicate 

 their cellular origin.* 



12 13 



14 



15 16 17 



The membrane constituting the tube is some- 

 times simple, like those of the simple cells : but 

 it frequently contains fibres, or other internal 

 coatings, corresponding to those met with in the 

 more compound cells. The vessels in which the 

 internal fibres run in a spiral direction (Fig. 14), 

 are denominated trachea, or spiral vessels; or, 

 from their being found very constantly to contain 

 air, they are often called air tubes. Their 

 diameter is generally between the 1000th and the 

 300th part of an inch. These spiral, or air ves- 

 sels, pervade extensively the vegetable system. 

 The threads they contain are frequently double, 

 treble, quadruple, or even still more numerous : 

 they are of great length, and when the external 

 membrane of the vessel is divided, they may 



* This theory of the derivation of vessels from cells was first 

 advanced by Treviranus. 



