VEGETABLE ORGANIZATION. 



73 



observable in the vessels of plants also indicate 

 their cellular origin.* 



12 13 



15 16 \7 





p 



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 iracliece, or spiral vessels; or, 

 from their being found very constantly to contain 

 air, they are often called ai?- 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. 



