LECT. III.] VASCULAR TEXTURE. 81 



vided by him into two species solitary and fasci- 

 culated. Those which he terms solitary, however, 

 are, in my opinion, improperly regarded as vessels, 

 being* merely oblong cells, simple receptacles of 

 secreted juices. 



2. The PERFORATED VESSELS are cylindrical 

 tubes, the sides of which are pierced with minute 

 perforations variously distributed. They may be 

 divided, according to the character of the 



j-i 



perforations, into two species ; a. Cribri- 

 form vessels, the perforations of which are 

 simple pores, arranged in parallel series, 

 transversely and equidistant over the 

 whole surface of the tubes (fig. */, e). 

 Mirbel denominates them "poreux." He 

 asserts that each perforation is surround- 

 ed with an elevated border (bourrelet 

 saillant), as represented in fig. e ; an 

 appendage which I have never been 

 able to perceive; and observes, that they 

 must not be regarded as continuous tubes, 

 as they often separate, join again, sometimes dis- 

 appear altogether, and always terminate in cellular 

 texture. They are found in the substance of roots, 

 in the formed wood of stems, branches, leaf-stalks 

 and the central ribs of leaves ; and are most 

 numerous in hard woods, as of the Oak and the 

 Chestnut. Their pores are so extremely small, 

 that, in order to perceive them, a thin longitudinal 

 VOL. i. c 



