CHAP. I. VASCULAR TISSUE. '23 



although formerly stated by Mirbel to be continuous with tlie 

 cellular tissue, is so little known, that the learned De Can- 

 dolle, in his Organograpliie^ published in 1827, remarks, 

 " Personne jusqu' ici n'a vu d'une maniere claire, ni I'origine, 

 ni la terminaison d'un vaisseau." (P. 58.) As doubts upon 

 these points arise from the extreme minuteness of the vessels, 

 and from the different degrees of skill that observers employ 

 in the use of the microscope, I can scarcely hope that any ob- 

 servations of mine will have much weight. Nevertheless, I 

 may be permitted to state briefly what arguments occur to 

 me in support of the definition of the spiral vessel as given 

 above. 



With regard to the presence of an external membrane 

 within which the spiral fibre is developed, it must be con- 

 fessed that direct observation is scarcely sufficient to settle 

 that point. It is easy to prove the existence of a membrane, 

 but it is difficult to demonstrate whether it is external or 

 internal with respect to the fibre. The best mode of examin- 

 ation is to separate a vessel entire from the rest of the tissue, 

 which may be done by boiling the subject, and then tearing it 

 in pieces with the points of needles or any delicate sharp 

 instrument : the real structure will then become much more 

 apparent than if the vessel be viewed in connection Axdth the 

 surrounding tissue. From some beautiful preparations of 

 this kind by Mr. Valentine and Mr. Griffith, it appears that 

 the membrane is external : in the root of the Hyacinth, for 

 example, the coils of the spiral vessel touch each other, 

 except towards its extremities ; there they gradually separate, 

 and it is then easy to see that the spiral fibre does not project 

 beyond the membrane, but is bounded externally by the latter, 

 which would not be the case if the membrane were internal : 

 a representation of such a vessel is given at Plate II. fig. 9. 

 Another argument as to the membrane being external may not 

 unfairly be taken from the manifest analogy that a spiral 

 vessel bears to that form of cellular, tissue (p. 11.), in which a 

 spiral fibre is generated witldn a cellule : it is probable that 

 the origin of the fibre is the same in both cases, and that its 

 position with regard to the membrane is also the same. 



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