809. ] 
iwell defined; and, during, their forma- 
on, the vessels of the bark are distinctly 
sible, as different organs; and had, the 
ne been transmuted into the other, their 
rogressive changes could not have es- 
raped Mr. Knight’s observation. This 
zentleman asserts, that the organization 
of the bark in other instances ~ does hot, 
im any degree, indicate the character of 
the wood. that is generated beneath. it: 
thus, the bark of the wych elm is ex- 
tremely tough and fibrous; that of the 
ash, at the same age, breaks almost as 
-readily in any one direction asin another, 
and presents very little of a fibrous tex- 
ture; yet the alburnum of these ‘trees is 
not very dissimilar, and the one is often 
substituted for the other in the construc- 
tion of agricultural instruments. 
Mr. Knight examines and controverts 
the theories of Mirbel and Duhamel. The 
latter has shewn, that when a bud of 
a peach tree, with a piece of bark at- 
tached to it, 1s inserted in a plum stock, 
a layer of wood, perfectly similar to that 
of the peach tree, will be found, in the 
succeeding winter, beneath the inserted 
bark; but this experiment does not prove 
the conversion of bark into wood; for 
* the probable operation,’’ according to 
Mr. Knight, “of the inserted bud, whicli 
is a well organized plant, at the period 
when it becomes capable of being trans- 
posed with success, appears to have been 
overlooked; tor I found that, when £ 
destroyed the bark which belonged to 
them uninjured, this bark no loager pos- 
sessed any power to generate” alburnum. 
Tt nevertheless continaed to live, though 
pertectly inactive, till ir became cov er 
by the successive albarnous layers of the 
stock ; and it was found, many years af 
terwards, Kaclosed 11 the wood. It was, 
however, still bark, though dry and life- 
less, and did not appear to have made 
any progress towards conversion into 
wood.” From these, and from various 
other experiments, made expressly tor the 
purpose, Mr. Kk. concludes, that bark is 
never transmuted into alburnum. 
In another paper our author maintains, 
that the bark deposits the alburnous mat- 
ter. In proof of which he says, if the 
succulent shoot of a horse-chesnut, or 
other tree, be examined, at successive 
periods in the spring, it w ‘ill be seen, that 
the alburnum is deposited, and its tubes 
arranged in ridges beneath the cortical 
vessels, and the number of these ridges, 
at the base of each leaf, will be found 
to correspond accurately with the num- 
ber of apertures through which the ves- 
Proceedings of Learned Societies. 
59 
sels pass from the leaf-stalks into the in- 
terior bark, the alburnous matter being 
apparently deposited by a fluid which de-. 
scends from the leaves, and subsequently. 
secretes through the bark. Hence it ig 
inferred, that ‘the alburnum is. thus de- 
posited; and an enquiry is instituted re- 
specting the origin and office of the-al-. 
burnous tubes. They have generally been 
considered as the passages through which 
the sap ascends, and, at their first forma- 
tion, they are always filled with the fluid, 
which has apparently secreted trom the 
bark. ‘They appear to be formed in the 
soft cellular moss, which becomes. the 
future alburnum, as receptacles of this 
fluid, to which they may either afford a 
passage upwards, or simply retain it as 
reservoirs, till absorbed and carried off 
by the surrounding cellular substance. 
From some decisive experiments Mr. 
K. thinks, that the sap does not rise 
through the tubes of the alburnum, but 
through the cellular substance; which, he 
thinks, may give the impulse with which 
the sap is known to ascend in the spring; 
and, if it be thus raised, much of it will 
probably accumulate in the alburnum in 
the spring ; because the powers of vege- 
table life are, at that period, more active 
than at any other season; and the leaves 
are not then prepared to throw off any 
part of.it by transpiration. And the 
cellular substance, being then filled, may 
discharge a part of its contents into the. 
alburnous tubes, which again become re= 
servoirs, and are filled to a greater or less 
height, in proportion to the vigour of the 
tree, and the state of the soil and season; 
and if the tubes, which are thus filled, 
be divided, the sap will flow out of them, 
and the tree will be said to bleed. But, 
as soon as the leaves are unfolded, aad 
begin to execute their office, tne sap will 
be drawn from is reservoirs, and the tree . 
will cease to bleed, if wounded. 
Mr. K. further observes, that the al- 
burnous tubes appear to answer another 
purpose in trees, and to be analogous, im 
some degree, in their effects, to the cavie 
ties in the bones of animals; by which 
any degree of strength that is necessary 
is given with less expenditure of mate 
rials, or the incumbrance of unnecessary 
weight ; and the woed of many different 
species of trees is thus made at the time 
yery hght, and very strong; the rigid ve= 
getable fibres being placed at greater dis- 
tances from each other by the interven- 
tion of alburnous tubes, and consequently 
acting with greater mechanical advantagé 
than they would if pet immediately ins 
contact 
