CHAP. 1, 
VASCULAR TISSUE. 
23 
although formerly stated by Mirbel to be continuous with the 
cellular tissue, is so little known, that the learned De Can- 
dolle, in his Organographies published in 1827, remarks, 
" Personnejusqu' 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 mvist 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 maybe 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 with 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 11. 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 ivithin 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. 
c 4 
