98 
BOYS DEPARTMENT. 
these medullary rays appear like mere lines running 
across the end, they in reality extend through the 
whole length of the trunk, dividing hoth heart and 
sap wood into distinct sections. 
The parts, or divisions, of the trunk, above de- 
scribed, are such as are apparent without minute 
observation ; but a closer inspection discloses small 
cells, or tubes, extending lengthwise through the 
trunk, and arranged in groups, or bundles. Through 
these channels the sap flows, passing in its ascent 
through the tubes of the alburnum, and in its de- 
scent through those of the liber and bark. In that 
portion of the trunk called the heart, the cells have 
become mostly filled and closed with solid matter, 
so that but little juice can flow through them. The 
heart, therefore, is the more firm and solid portion, 
and on that account the more valuable for mechani- 
cal purposes where strength and durability are re- 
quired. In all ex-tropical trees, the palm family 
excepted, a new layer of wood is formed every year 
next to the bark, which forms a ring around the 
trunk, easily recognized in most trees when cut or 
sawed across ; their age may be readily ascertained 
by counting these rings, or layers. 
3. The Leaves. — These are very important organs, 
as they prepare the juice for the office it performs 
in forming new wood, and thus promoting the 
growth of the tree. Their office in this respect 
may be compared to that of the lungs of animals, 
for the lungs in like manner prepare the blood for 
its destined purpose. 
The leaf is connected both with the- wood and 
the bark of the twig to which it is attached ; its 
fibres communicating with the woody part of its 
stem, while the green part of the leaf may be con- 
sidered as an expansion of the bark. The tubes, or 
cells, which I have described as extending through 
the alburnum and liber, pass into the leaf and dif- 
fuse themselves through its substance. Now 
there are two sets or layers of these vessels, the 
one spread along the upper, the other along the 
under surface of the leaf. Those along the up- 
per surface come from, and connect with, the wood 
or alburnum, and those along the lower surface 
connect in like manner with the vessels belonging 
to the bark or liber. The vessels, running through 
the leaf communicate with the air by means of in- 
numerable pores which permit the escape of vapor, 
or admit the entrance of carbonic acid, as the case 
may require. It is supposed, (I know not on what 
authority,) that the office of the pores on the upper 
surface is to allow vapor to escape, while the ab- 
sorption of carbonic acid, (and perhaps other gases,) 
is performed by the pores of the lower surface. 
The Course of the Sap. — The juice, when first 
absorbed by the roots, is unfit for the purpose of 
nutrition. Its upward passage, as I have before 
said, is through the tubes of the sap wood, and 
these tubes are connected with those passing into 
the leaf. The sap in its ascent is slightly changed, 
though not materially, until it reaches the leaf, 
when it is exposed to the action of light and air, 
and reduced in quantity by the evaporation of a 
part of the water it contained. It is made thicker 
and somewhat mucilaginous or gummy, by parting 
with a portion of its water. Its chemical charac- 
ter is also changed by the action of the atmosphere 
as well as by that of light ; carbonic acid is drawn 
from the air, and it returns through the liber, carry- 
ing with it a large portion of this gas. Now, in 
what manner the carbon is separated, or how it is 
that those chemical changes are performed by which 
the materials contained in the juice are converted 
into wood, fruit, or seed, I cannot inform you. 
Some may satisfy themselves by saying these chan- 
ges are produced by a living principle in vegeta- 
bles, but this explanation is far from being satisfac- 
tory, and leaves us in doubt and darkness as to the 
nature of this principle, and the manner in which 
it acts. But what causes the motion of the juice 1 
Why does it rise contrary to the force of gravity % 
This question concerning the motion of juices in 
vegetables, like that of the circulation of blood in 
animals, has given rise to much speculation, and to 
many theories. One method of explaining the as- 
cent of sap is, by supposing it to be the result of 
capillary attraction ; another, and the one adopted 
by Liebig. (See American Agriculturist, vol. vii, p. 
276,) is that the evaporation' from the surface of 
the leaves produces a vacuum, (an empty space,) 
in the upper part of the tubes, which causes the 
fluid to rush in from below on a principle similar 
to the one by which water is forced upward in a 
pump ; others again have resorted to the vital prin- 
ciple, (a term hard to define,) but there are weighty 
objections to all these theories. The one adopted 
by Johnston appears more satisfactory, although it 
must be admitted that this rather illustrates than ex- 
plains the fact — just as the falling of an apple 
illustrates without explaining the force of gravity. 
It is as follows : — 
Take a metallic tube open at both ends ; over one 
end tie a piece of bladder, and into the other pour 
some water saturated with sugar, and immerse the 
lower end, (that over which the bladder is tied,) in 
some pure water. The water will penetrate the 
bladder, rise in the tube, and run out at the top, and 
continue running out until the water within, and 
that without the tube contain nearly the same pro- 
portions of sugar. Now sugar is a common ingre- 
dient in the juices of plants, and the above experi- 
ment may be made with several other substances 
which are found in vegetable juices. Is it not, 
then, highly probable that those liquids which the 
roots of plants imbibe are drawn up through their 
small tubes on this principle ? Neither capillary 
attraction, nor the production of a vacuum by evap- 
oration, nor the indescribable vital-principle hypo- 
thesis will account for the ascent of the water in 
the tube in the above experiment. Yet, while we 
admit that the chemical force by which the water 
is made to ascend in the tube is the principal one 
in causing the ascent of the sap in trees, I do not 
know that there is anything unreasonable in sup- 
posing that it may be aided in plants by capillary 
attraction, and also by evaporation from the leaves. 
This indeed seems quite probable when we consider 
that water can only be made to rise a few feet in 
our metallic tube, while in trees it is sometimes 
drawn upward of a hundred feet or more. 
In my next letter, I will describe some of the 
most important of the compounds belonging to 
vegetables. J. Mc Kinstry. 
Greenport, Columbia Co., N. Y. Feb. 1st, 1849. 
