64 



THE TMECHANICAL FUNCTIONS. 



The membrane constituting tlie tube is sometimes 

 simple, like those of the simple cells : but it fre- 



12 13 



15 16 1/ 



1/ 



\^s 



4 



-^ 



quently 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 deno- 

 minated trachece or spiral vessels; or, from their 

 being found very generally to contain air, they are 

 often called air tubes* Their diameter is in general 

 between the 1000th and the 300th part of an inch. 

 These spiral, or air vessels, pervade extensively the 

 vegetable system. The threads they contain are fre- 

 quently 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 easily be drawn out and uncoiled, their elas- 

 ticity enabling them to retain their spiral shape. 

 The object of this structure appears to be that of 

 keeping the cavity of the tube always pervious, by 



* Schleiden observes, however, that spiral vessels and spiral cells 

 occur in the living plant quite as frequently filled with sap, in the 

 young vegetating portions, as with air in the older organs which 

 have attained their full dimensions. 



