230 THE SOLID COMPONENTS OF PLANTS. 



an elevated border ; and observes, that they must not 

 be regarded as continuous tubes, as they often sepa- 

 rate, joiu again, sometimes disappear altogether, and 

 always terminate in cellular texture. They are found 

 in the substance of roots, in the formed wood of sterns, 

 branches, leaf-stalks, and the central ribs of leaves ; 

 and are most numerous in hard woods, as of the Oak 

 and the Chesnut. Their pores are so extremely small, 

 that, in order to peceive them, a thin longitudinal slice 

 of the wood to be examined must be cut, and placed 

 in a drop of pure water under a powerful microscope. 

 It has not been accurately ascertained what kind of 

 fluid is contained in these vessels. 



A modification of the perforated vessels has the 

 appearance of a string of beads, consisting, as it were, 

 of united portions of a porous tube, narrowed at the 

 extremities, and divided from each other by perforat- 

 ed diaphragms. This variety of perforated vessels 

 is found frequently in roots, and at the going off* 

 of branches, and the attachments of leaves, being, 

 says Mirbel, " intermediate between the large ves- 

 sels of the stem and those of the branches ; and it 

 is by their means," as he conceives, " that the sap 

 passes from the one set of vessels into the other."* 



Another variety of the perforated vessels, called 

 annular, are so named from the perforations being 

 transverse and oblong, as if the tube were formed of 

 rings, of the same diameter, placed one above another, 

 and attached at some part of their edges, but not touch- 

 ing throughout the whole circumference. These are, 

 in fact, porous vessels, with oblong transverse perfo- 

 rations, resembling in every respect, except shape, the 

 round pores of the last described vessels. They are 

 also surrounded by a border, and convey resinous and 



* Elem. de Phys. Veget. lere Partie, p. 31. 



