176 
THE CULTIVATOR. 
the cells, as water is diffused through blotting paper 
or lump sugar. These fluids, as soon as they enter 
a plant, whatever may be the manner in which this 
is effected, take the name of sap, the rise of which 
into the stem is demonstrated by the ingenious expe¬ 
riment of watering with very thin coloured fluids, 
some of which are taken up, though the ordinary sap 
is for the most part without colour, and transparent 
like water. The yellow juice of celandine, and the 
milky juice of spurge are not sap. 
In animals, the nutritive portion of the digested 
food which is taken up, as we have already seen, on 
the inner surface of the intestines, by the mouths of 
innumerable small tubes, is thence carried by the 
union of these into one* * * * § of considerable size, which 
conveys the whole into a vein,f where it is mixed 
with the blood. It is thence carried to the heart, by 
whose motion or beating it is thrown into a large 
vein,! and carried to the lungs, where it is distributed 
into innumerable small tubes and exposed through 
their very thin membranes to the fresh air taken in 
by breathing. Here the blood takes from the air a 
portion of its oxygen, and at the same time parts with 
a quantity of carbonic acid gas, which is carried off 
by the returning breath: hence deteriorated by the 
loss of oxygen, and by having become loaded with 
carbonic acid. It is this deterioration, which renders 
it indispensable for all animals to breathe fresh air, as 
confined air is soon rendered poisonous and becomes 
fatal. The blood thus freed from superabundant car¬ 
bonic acid, and united with a portion of oxygen, is 
collected by innumerable minute arteries which unite 
into one,5 and carry it back to the heart, to be thence 
distributed to every part of the body, for the purpose 
of supplying fresh materials for the growth or repair 
of the several parts. 
These things I have stated more at length, in order 
to point out the very different processes that take 
place in plants, which have nothing like the animal 
heart to impel the sap into circulation, for the pith in 
the centre of trees, though popularly termed the heart, 
performs no such office. 
USE OF LEAVES. 
The leaves, however, have been proved, by nume¬ 
rous experiments, to be in some respects similar in their 
functions to the lungs of animals, though very unlike 
if considered-As organs. The sap, for instance, is 
spread out in k leaf, into small portions, as is the 
blood in the animal lungs; and for the similar pur¬ 
pose of exposing it to the action of the air. 
It is important to remark that the air acts on the 
blood in the animal lungs always in the dark, where¬ 
as in acting upon the leaves of plants, it is by turns 
in the dark, and by turns in the light—and the results 
under these two circumstances are very different. 
In the dark, leaves, like the lungs of animals, take 
in oxygen from the air and part with a portion of the 
carbonic acid gas contained in the sap; from which 
it is evident that they obtain a greater portion of this 
important material than is supplied by the water or 
the atmospheric air mixed with it, taken in by the 
roots from the soil. 
It follows, that the air of a green-house, or any 
room or confined place where plants are kept, must 
be unwholesome during the night, from the air in 
such cases being deprived of its oxygen, and also load¬ 
ed with carbonic acid gas. Consequently, flowers 
ought not to be kept at night in bed-rooms, though 
during sun-light they purify the air. 
In the light, the sap which is spread over the up¬ 
per surface of a leaf immediately under the thin, co¬ 
lourless, and transparent skin,|| parts with a portion 
of its water in the form of vapor, and also with the 
oxygen contained in the carbonic acid gas, (which is 
composed of oxygen and carbon,) and as the oxygen 
goes off the carbon remains, while the sap, previously 
little less fluid than water, is converted into a sort of 
pulp, IT a considerable proportion of which consists of 
carbon. The use of this pulp, as we shall see anon, 
is, like the animal blood, to supply fresh materials, for 
the growth of the plant; and as the blood, after it 
has been acted on by the air in the lungs, passes 
from the veins to the arteries, so the pulp of plants is 
plausibly conjectured, but not proved, to pass from 
the upper to the under side of the leaf. The tubes 
or cells in which this pulp is lodged being of a yellow 
colour, and the carbon, according to Sennebier, of a 
dark blue, produce between them the green colour of 
the leaves and the younger bark, and hence the dark¬ 
ness of the green becomes a sort of test to determine 
the quantity of carbon separated from the sap, and 
also in some measure of tenderness and strength; 
plants being usually vigorous, hardy and tough, in 
proportion to their carbon, or, if Sennebier is right, in 
proportion to the darkness of their green. The term 
* Technically, Thoracic Duct. 
f Technically* Subclavian Vein. 
; Technically, Pulmonary Vein. 
I Technically, Pulmonary Artery. 
Technically, Cuticle; in Latin, Cuticula. 
f In Latin, Cambium, formerly Parenchyma. 
vigorous appears to be misapplied to plants of a pale 
colour, which grow too large and become dropsical. 
It is, at all events, certain, that blanched plants are 
tender; that bulbs placed so deep as to blanch a 
large portion of their leaves before reaching the sur¬ 
face, are too weak, as I have experienced, to flower 
well ; and that young leaves in the spring, before 
they acquire their full green, are readily injured by 
frosts and cold winds. 
In accordance with these views, it may be stated 
that plants are of darker green in Italy, where the 
sun is seldomer overclouded, than in England ; and 
though the grass has certainly a good colour in 
“green Erin,” it is not, I think, to be compared with 
the beautiful green meadows in the Canton of Berne 
in Switzerland, where the sun-light is more intense 
from the general elevation of the country. In Ame¬ 
rican forests, when the young spring leaves appear, 
they sometimes continue pale yellow for several 
weeks, from the haziness of the sky, but no sooner 
does the haze clear away, and the sun shine out, 
than the green deepens almost visibly. “ The colour 
of the forest,” says an eye-witness, “ absolutely chang¬ 
ed so fast, that we could perceive its progress ; by 
the middle of the afternoon, the whole of these exten¬ 
sive forests, many miles in length, presented their 
usual summer dress.” 
It is a very curious fact connected with this sub¬ 
ject, that, as it is on the upper surface of the leaf only 
where the sap is changed into pulp by the action of 
air and light, the leaf cannot be reversed, so as to 
place its upper surface undermost, and away from the 
direct light of the sky, without injury. When Bon¬ 
net accordingly placed leaves with their upper sur¬ 
faces upon water, they withered almost as rapidly as 
in dry air. 
Some practical gardeners, such as Lyon and Hay¬ 
ward, are disposed to question this circulation of the 
sap, but it has been proved beyond all doubt by expe¬ 
riment. Dr. Darwin plunged plants of spurge in wa¬ 
ter tinged red, and saw it rise through the leaf-stalk, 
and after being decomposed by the air and light, saw 
it return white from the edges ot the leaf. T. A. 
Knight traced the returning pulp back into the leaf¬ 
stalk, and thence into the inner bark. He found also 
that if the leaves be stript from the upper part of a 
branch, the bark will wither as far as it is stript; and 
if a ring of the bark be cut out above and below a 
leaf, the wood in the part above the leaf does not 
grow. 
It is therefore fair to conclude, that all the increase 
in growth arises from the leaves, that is from the 
pulp, prepared in the leaves by light and air, in the 
same way as all the increase in the growth of animals 
arises from the blood prepared in the lungs by the air, 
without light. 
The doctrine held in some recent works of high pre¬ 
tensions, that the water given off by a plant is not 
produced “exclusively by the action of light and air,” 
but “ is due* to evaporation, or prespiration,” which 
sometimes causes water equal in weight to that of 
the plant to be perspired in twenty-four hours, is quite 
erroneous. It is altogether owing to light or air. 
The high importance of the leaves becomes thus 
manifest, and nothing wdl more enfeeble a plant than 
taking off its leaves in the growing season ; though 
they are no longer necessary during the cessation of 
growth in the winter. Their fall previous to winter 
is not caused by cold; for some trees drop their leaves 
while the weather is comparatively warm, but in con¬ 
sequence of the vessels at the root of the leaf-stalk 
becoming gradually rigid and obstructed, so as to pre¬ 
vent the rise of the sap, or, at least, the return of the 
pulp. When a branch is in any way killed during 
the summer, its leaves do not fall, so that the killing of 
a leaf will not make it fall. 
The slower growth of evergreens, and the diffe¬ 
rence of their pulp, as well as of the texture of their 
skin, render their leaves less subject to a general fall, 
which takes place only partially and by degrees. It 
is probably on account of the skin of the leaves be¬ 
coming thicker and firmer from exposure to brighter 
sunshine, that our oak and other similar trees become 
evergreens in warm climates, as St. Helena. 
DESCENT OF THE PULP. 
That the pulp formed from the sap in the leaf, pas¬ 
ses back through the leaf-stalk into the bark, has just 
been proved ; but the best inquirers are not agreed 
upon the manner in which it is afterwards distributed 
so as to nourish all parts of the plant. It is sufficient 
to know the unquestionable fact that it does so. A 
portion of it certainly passes down through the bark 
to the very root, where it in all probability throws 
out, as would appear from the experiments of Brug- 
mans and Mirbel, such refuse materials as could not 
be used, and something of this kind may be seen ad- 
hereing to the root fibres of hyacinths growing in 
water-glasses. 
A portion of it, in its descent, must also pass off in¬ 
* Due, Anglo-French for “ owing,” 
to the stem, probably, by what may be termed pulp- 
cells, or by rays or plates, popularly termed the silver 
grain.* The pulp-cells have been supposed to be de¬ 
signed as reservoirs for spare nutrient matter, some¬ 
what like the fat of animals, which is similarly stored 
up. 
SEASONS OF GROWTH AND ITS CESSATION. 
Too little attention has been paid in books of sci¬ 
ence to the different circumstances of plants, during 
the several seasons of the year ; and not a few errors 
have thence been the consequence. It is commonly 
stated, for example, that “the sap rises in spring and 
descends again in autumn,” whereas we have seen 
that the sap does not descend at all, at least in the 
state of sap, and it is very clear that in proportion as 
the pulp-vessels in the leaf-stalks become obstructed 
with woody or carbonaceous matter towards the au¬ 
tumn, a circumstance indicated by the leaves becom¬ 
ing tinged with yellow, the quantity of descending 
pulp must be diminished, and the rise of the sap, from 
the same cause, must be interrupted. 
The facts then, so far as at present ascertained, 
appear to be, that, from this obstruction in the sap 
and pulp-vessels of the leaf-stalk, the sap rises in gra¬ 
dually diminished quantities, till, at length, when the 
leaves fall, it almost ceases. I say “ almost,” for that 
it does not altogether cease to rise is proved by the 
buds and bark continuing alive and fresh, though they 
would infallibly wither and die, if the supply of sap, 
admitted to be very small, but indispensable to their 
healthy state, were cut off by removing the roots. 
Certain animals which cease to eat on the setting 
in of winter, such as the squirrel, the bat, the snake, 
the frog, the snail, and numerous species of insects, 
continue to live in a sluggish and nearly motionless, 
state, very different from sleep, in the circumstances, 
particularly, of the very slow and scarcely observable 
circulation of the blood, and of breathing, whose ra¬ 
pidity is not very greatly affected during ordinary 
sleep. 
The terms dormancy and dormant, therefore, are 
inaccurate, when applied to this state, which is better 
designated by torpidity and torpid. Such animals 
are found previous to winter to become unusually fat, 
as indeed is the case, at this season, with most other 
animals, owing in part to the loss of substance be¬ 
coming less, as perspiration diminishes with the in¬ 
crease of the cold. 
Now plants, in winter, seem to be in circumstances 
very similar to those ot torpid animals, the circulation 
of the sap, like that of the blood, and the changes ef¬ 
fected by air and light, like the process of breathing, 
being scarcely, if at all, observable. With respect 
also to the increase of fat, it is said by T. A. Knight, 
De Candolle, and others, that a larger proportion 
than usual of nutrient pulp accumulates in the root, 
and in the pulp-wood.f That this is the case with 
the buds of trees, with the bulbs of bulbous plants, 
and with the crown! of the root of most perennial 
plants, and with the tubers of potatoes and dahlias, I 
think there can be no question. 
In this state of torpidity, then, the greater number 
of plants continue during the winter ; but the torpidi¬ 
ty is more or less in degree according to the mildness 
or severity of the weather. I gathered a wild rose,§ 
fully blown, in Musselburgh Haugh, in. Dec. 1818; 
and if the branch of a vine whose root is planted in 
the open air, be introduced in winter into a green¬ 
house, it will push out leaves, while the branches 
without remain torpid. That it does not derive its 
nourishment from the air of the green-house, was 
proved by De Candolle, who found that the roots of a 
vine thus forced into leaf, exhausted the water of a 
bottle in which he placed them, while those of an ad¬ 
jacent vine not thus forced into leaf, took up very lit¬ 
tle, if any, water from a bottle. 
Ao animals, revived from their winter torpidity by 
unusually warm weather, are more apt to be injured 
by the recurrence ot severe cold than if they had 
continued torpid, a circumstance but too frequently 
witnessed in the case of hive-bees, so plants prema¬ 
turely roused from their natural winter torpidity by 
warmth, to expand their buds, are almost certain to 
suffer by it, should severe weather afterwards ensue. 
It is important to remark here, that, like the leaves, 
the smaller root fibres shrivel and fall off, or become 
detached from the main root, a circumstance, which 
has not hitherto received the attention which it well 
merits, both in a scientific and in a practical point of 
view. The fact is very obvious in the potato, the ra- 
nunculis and anemone, the dahlia, the carmyle,|| and 
in all bulbs. 
“The soil,” says De Candolle, “is warmer than 
the air at mid winter(being always about eight 
degrees above freezing;) “ this warmth rouses into 
life the trunks and roots then full of the nutrient pulp 
* In Latin, Rudd or Laminae, medullares. 
f In Latin, Alburnum, 
j In Latin, Corona, or Torqua. 
§ In Latin, Rosa spinosissima. 
| In Latin, Orobut tuberosus. 
