Pe 
* 
+ 
yo 
call 
54 Vegetable Organography and Physiology. oe. 7 
of vegetable life, and analogous to the nervous system in astral 
is still an interesting question for the vegetable physiologist. 
Immediately surrounding the pith is the medullary sheath. 
Whilst the former is composed of cellular tissue, the latter consists 
of spiral vessels and ducts. The office of the medullary sheath 
is not well understood. Lindley, who believed it to be in direct 
communication with the leaf-buds, and the veins of the leaves, 
supposed that it was the medium through which the ascending 
sap is transmitted to the leaves. Dutrochet and some other phys- 
iologists are of the opinion that the office of circulating the as- 
cending sap is confined to the lymphatic tubes of the woody fibre, 
and that the spiral vessels of the medullary sheath belong to the 
function of respiration. As these vessels are found almost invaria- 
bly to contain air; and, moreover, have direct communication 
with the leaves, which are in fact the lungs of the plant, it is 
_more than probable that the opinion of Dutrochet is correct. 
¢ _ Deposited in concentric layers around the medullary sheath, 
and lying immediately upon it, is the wood. It is composed of 
cellular tissue, woody fibre, and ducts. Each year a distinct layer 
_is deposited. ‘The concentric layer of the first year, or the one 
~ lying i in immediate contact with the medullary sheath, consists of 
woody fibre and ducts ; but each succeeding layer has an interior 
membrane of cellular tissue, (the same tissue of which the pith is _ 
ot Eaxaponed, ) and an extemal stratum of woody fibre and ducts. 
‘wood is usually subdivided into the denser portion, which 
_ is called gnum ; and external to this, a softer portion called a+ 
- burnum. 
The former consists of the internal avon surrounding the me- 
dullary sheath, which being fully formed, have ceased to afford 
a passage for the circulating fluid. It is aba called heart-wood. 
External to this is the softer wood, called the alburnum ; whie 
is also deposited in concentric rings, between the true weal and 
the bark. It is through this part of the plant, the alburnum, that 
the fluid drawn from the earth, the ascending sap, is principally 
transmitted. he bark is composed of the same elementary tl 
a that enter into the composition of the wood, viz. cellular tis- 
sue, woody fibre and ducts ; but whilst the layers of wood con- in 
- sist ‘of an interior stratum of cellular tissue, and an outer stratum 
of woody fibre and ducts, those of the bark are reversed: they a 
composed of a layer of woody fibre and ducts inside, and of cellu- 
