10 
ORGANOGRAPHY. 
BOOK I. 
while by some it has been supposed to be formed by the 
doublings and foldings of a membrane in various directions. 
On both these suppositions, the partitions between the cells 
would be simple, and not composed of two membranes in a 
state of cohesion ; but the facility with which, as has just been 
stated, the cellules may be separated, sufficiently disproves 
these opinions. But although the double nature of the par- 
titions in cellular tissue may be often demonstrated, yet the 
cellules usually grow so firmly together, that their sides really 
form in their union but one membrane ; and it will be here- 
after seen that in many cases the partition between two 
cells is originally simple. 
The bladders of cellular tissue are destitute of all perfora- 
tion or visible pores, so that each is completely closed up from 
its neighbour, as far as we can see ; although as they have 
the power of filtering fluids with rapidity, it is certain that 
they must abound in invisible pores, and that they are not 
impermeable, as if they were made of glass. An opinion 
different from this has been entertained by some observers, 
who have described and figured perforations of the mem- 
brane in various plants. Mirbel states that “ the sides of the 
bladders are sometimes riddled full of holes [Jig, 4.), the 
fig. 4 . aperture of which does not exceed the 3 ^^ 
of a millimetre (or of half a line) ; or are less 
frequently pierced with transverse slits, which 
are occasionally so numerous as to 
transform the bladders into a real 
articulated tissue, as in the pith of 
the Nelumbium [Jig. 5.).” This 
statement is now well known to have 
been founded upon inaccurate observation ; what 
the supposed pores really are has already been explained. 
(See p. 5.) 
With reference to this subject, it may be also observed, that 
the bladders often contain air-bubbles, which appear to have 
no direct means of escape, and that the limits of colour are 
always very accurately defined in petals, as, for instance, in 
the stripes of tulips and carnations, which could not be the 
case if cellular tissue were perforated by such holes as have 
