THE SOLID COMPONENTS OF TLANTS. 229 



and in the diameter of their calibers. They are com- 

 posed of the membranous texture, are firm, compara- 

 tively thick, and somewhat pellucid. Mirbel describes 

 6 different kinds of vessels ; but the whole may be 

 arranged under the 3 following genera : viz. 1st. En- 

 tire vessels ; 2d. Perforated vessels •, 3d. Spiral ves- 

 sels. 



1st- The Entire vessels are, as their names im- 

 port, simple tubes formed of imperforated membrane. 

 They are cylindrical ; and are generally in bundles, 

 regularly disposed in the cellular part of the bark. 

 They are found in the young shoots of almost every 

 kind of plant ; and in the fasciculated state may be 

 readily detected, and examined by the aid of mag- 

 nifying glasses, in the leaf-stalk of the common Fern, 

 in the Arrow-head (Sagittaria sagittifolia), and in the 

 Hemp plant. In order to examine them individually, 

 the bundles should be steeped in spirits of turpentine 

 for a few days, by which means the vessels can be 

 easily detached from one another. 



These vessels are intended to convey the proper 

 juices of the plant, and are generally found filled with 

 oils, and resinous juices ; consequently they are more 

 numerous in plants, the juices of which are of a thick 

 resinous nature ; and these drying along with the con- 

 densed vessel in the bark, are the matters on which the 

 medicinal virtues of barks in general depend. 



2d. The Perforated vessels are cylindrical 

 tubes, the sides of which are pierced with minute per- 

 forations variously distributed. They may be divided, 

 according to the character of the perforations, into 2 

 species ; viz. Cribriform vessels, the perforations of 

 which are simple pores, arranged in parallel series, 

 transversely and equidistant over the whole surface of 

 the tubes. Mirbel denominates them porous vessels, 

 and asserts, that each perforation is surrounded with 

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