474 Reseurches into the Anatomy of Plants. 
structure is well known to every botanist, and as no vessels 
enter them, secretion and excretion must be performed by 
filtration. 
The hairs or down are always well known. From some of 
them oozes a viscous or resinous liquor, and there are others 
(those in the roots for instance) which seem destined to 
suck up moisture. 
VI. The Structure of the Stalk. 
The following are the varieties in the structure of the 
stalk of several families of plants : 
1. The stalk consists entirely of parenchyme in which 
we find scattered pieces of wood: there is therefore no 
bark or pith. Gramina, Cyperacez. 
I denominate parenchyme every kind of cellular, and I 
call wood a mixture of fibrous vessels and of sap. 
2. A layer of parenchyme forms the external bark, a layer 
of fibrous vessels the interior. The rest of the stalk is 
composed of parenchyme, in which the parcels of wood 
are dispersed, There is a bark therefore, but no pith. 
Liliacew, Cucurbitacea. 
3. The external bark is formed by the parenchyme, the 
internal bark by the fibrous vessels, and the texture of soft 
wood: a layer of wood surrounds the pith. Most of the 
dicotyledontal plants. 
4. As in No. 3, but in the pith there are scattered pieces 
of wood, 
5. Bark internal and external: the rest of the stalk com- 
posed of parenchyme, the middle of which is occupied by a 
piece of wood. Some Naiades and other aquatic plants. 
Ferns, 
In order to comprehend thoroughly the structure of the 
stalk we must trace its growth. In PI. II. tig. 12. we see 
a piece of a young branch of the Platanus orientalis, cut 
transversely. The interior is composed of parenchyme in 
which we find a circle of packets of wood separated from 
each other. The bark, letter a, is still in a free communi- 
tion with the pith d, by the intervals between the pieces of 
wood, letter e. The pieces of wood consist as usual of 
fibrous vessels, Jetter J, and of trachee, letter c. When 
we cut the same branch lengthways, we see all the parts 
more distinctly./ In fig. 14, the bark is represented at a, 
the vessels are exhibited at J, the trachez at c, and the pith 
occupies the middle at d. The woody pieces are very large, 
they consist of fibrous vessels at ), mixed with false trachez 
or porous tubes at c*, the trachese are atc. By their swel- 
ling they have compressed the parenchyme which separated 
them, 
