8 
ORGANOGRAPHY. 
BOOK I. 
There is no doubt that all these forms are in reality 
modifications of one common type, namely, the simple cell, 
(according to Morren of an amylaceous granule) however 
different they may be from each other in station, function, or 
appearance. For, in the first place, we find them all deve- 
loped in bodies that originally consisted of nothing but cel- 
lular tissue; a seed, for instance, is an aggregation of cells 
only ; after its vital principle has been excited, and it has 
begun to grow, woody tissue and vessels are generated in 
abundance. We must, therefore, either admit that all forms 
of tissue are developed from the simple cell, and are conse- 
quently modifications of it; or we must suppose, what we 
have no right to assume, that plants have a power of spon- 
taneously generating woody, vascular, and laticiferous tissue 
in the midst of the cellular. Mirbel has lately reduced the 
first of these suppositions to very nearly a demonstration ; in 
a most admirable memoir on the development of Marchantia 
he speaks to the following effect. ‘ I at first found nothing 
but a mass of tissue composed of bladders filled with little 
green balls. Of these some grew into long slender tubes, 
pointed at each end, and unquestionably adhering by one of 
their ends to the inside of the sac ; others from polygons 
passed to a spherical form in rounding off their angles. As 
they grew older, other very important changes took place in 
certain cells of the ordinary structure, which had not pre- 
viously undergone any alteration : in each of these there 
appeared three or four rings placed parallel with each other, 
adhering to the membrane, from which they were distinguished 
by their opaqueness ; these were altogether analogous to an- 
nular ducts. The cells become tubes did not at first differ 
from other cells in any thing except their form ; their sides 
were uniform, thin, colourless, and transparent ; but they 
soon began to thicken, to lose their transparency, and to be 
marked all round from end to end with two contiguous 
parallel streaks disposed spirally. They then enlarged, and 
their streaks became slits, which cut the sides of the tubes from 
end to end into two threads, whose circumvolutions separated 
into the resemblance of a gun-worm.’ In these cases there 
can, I think, be little doubt that the changes witnessed by 
