TISSUE OF VESSELS. 



71 



pressed into this form by the sap arising between the 

 pith and the bark. The silver grain appears to keep 

 open the jcommunication between the bark and the 

 pith, which the formation of the wood would other- 

 wise have separated. 



Slices of plants, magnified a, a, the bark ; b, b, the pith ; c, c, 

 rays of the silver grain. 



TISSUE OP VESSELS. 



IF the opinion of Kieser, Link, and some other 

 liigh authorities be correct, that the fibres and straight 

 vessels just described, originate in what are termed 

 spiral vessels, it would have been proper to begin with 

 these ; but as this is a point still doubtful, though 

 not improbable, I have preferred the present arrange- 

 ment. 



The Spiral Vessels. 



These vessels ] are found coiled up like a corkscrew ; 

 or rather resembling a wire as small as a hair wound 



(!) In Latin, Vasa spirulia ; by some, Trachea, 



