Researches into the Anatomy of Plants: 475 
them, and have left obscure lines at e, which form what 
may be called the radii of the wood, and which Grew 
called medullary insertions. Some authors have attributed 
essential functions to these insertions: we see clearly that 
here they are merely compressed parenchyme. 
In order still better to elucidate all these parts, I cut the 
same branch lengthways along the woody parcels. We 
then see (fig. 15, letter a,) the fibrous vessels of the wood 
at b, and the false trachez or porous tubes at c*, I then 
made another section. I cut the branch of the half of the 
wood to the pith by an obscure line lengthways. In this 
way I most distictly saw the compressed parenchyme, fig. 
16, lettere: the trachez at c and the pith atd. Finally, 
I made a third section ; I removed a piece of the surtace of 
the wood in order to see the parenchyme, Pl. J. fig. 13, 
Jetter J, which is insinuated between the fibrous vessels, 
letter a. 
Such is the structure of the stalk in almost all the dico- 
tyledontal plants. The stalk contains parcels of wood ar- 
Tangéd in a circle, these approach each other in growing, 
they form an entire layer, they compress the parenchyme 
which separated them, in this way they form the radii 
which we see on cutting the wood horizontally. 
It might be supposed that the parcels of trachez, fig. 19, 
letter c, are pushed towards the middle, fig. 13, letter c, by 
the increase of the packets of wood. But the pith is di- 
minished by the growth of the stalk without undergoing 
any compression, which must take place, if the trachee 
were pushed towards the pith. The cellules of the pith are 
larger in old than in young stalks. It is probable therefore 
that new parcels of trachez surrounded by fibrous vessels, 
are formed in the parenchyme of the pith, and that they 
have compressed the latter towards the sides, without ex~ 
ercising a pressure towards the centre; and consequently, 
that the tracheze have been chartged into false trachea, or 
into porous tubes. 
We know that the bark is separated from the wood, at 
the commencement of summer, and that it remains attached 
to it during the rest of the year. This separation takes 
place in the wood itself, Pl. IT. fig. 13, letter f, where we 
see the first porous tubes. It does not take place in herbs 
or in young branches. When we separate the bark in the 
Jatter, we only take off the layer of parenchyme which 
forms the external bark. Jt ig certain the trachez or porous 
tubes contribute to this separation; because we do not find 
any in the internal bark which is composed of fibrous ne 
sels 
