18 
along the upper surface of the leaf without wet- 
ting it, or lodge in its folds like globules of quick- 
aikiae So sila f in the case of fruits covered with 
bloom. It is probable, therefore, that all such 
leaves and vegetable surfaces as repel moisture 
are fitted rather for the inhalation of air, which 
they have long been regarded as capable of effect- 
ing; and in times in which it was fashionable to 
look for analogies between the plant and animal, 
in everything whatever, leaves were even semen 
ed as being the lungs of plants. Grew thought 
he had discovered in the leaves a number of lit- 
tle bags or bladders filled with air. The air was 
supposed to have entered by inhalation, and the 
bags or bladders were supposed to be analogous 
in their function to the cells of the lungs of ani- 
mals. M, Papin introduced into the receiver of 
an air-pump an entire plant, root, stem, and 
leaf; but the consequence was that it very soon 
ied He then introduced a plant by the root 
and stem only, the leaves being still exposed to 
the influence of the air. The plant lived fora 
considerable length of time, and hence he con- 
cluded that leaves are lungs. But these facts 
are far from being sufficient to settle the point 
in question, and we introduce them, not so much 
with a view to show their inadequacy, as to show 
that the doctrine, even if founded in truth, could 
not have been satisfactorily demonstrated by any 
experiments that were practicable at that time. 
It is to the modern improvements in pneumatic 
chemistry, and to them alone, that we are in- 
debted for our knowledge of the real functions of 
the leaves of plants, and of their analogical re- 
semblance to the lungs of animals; it being now 
proved indisputably that the tare of plants not 
only contain air, but do both inhale and respire 
it. It was the opinion of Priestley that they in- 
hale it chiefly by their upper surface ; and it has 
been shown by Saussure that their Sian Tba g power 
depends entirely upon the integrity of nae or- 
ganization. A bough of Cactus opuntia, de- 
tached from the plant, and placed in an atmo- 
sphere of common air, inhaled in the course of a 
night four cubic mene of oxygen; but when 
placed i in a similar atmosphere, after being cut to 
pieces and powndeG in a mortar, no inhalation 
took place.” 
M. Adolphe Brongniart, in a memoir upon the 
structure of leaves, and on their relation with 
the respiration of vegetables in air and water, 
read before the Academy of Sciences in Paris, in 
1830, states that the leaves of plants that live in 
the air have a totally different structure from 
those that are completely submerged, and that 
this difference in the structure of organs is in 
direct relation to the two particular functions of 
leaves, respiration and transpiration. In leaves 
exposed to air, the surface of the leaf is covered 
by an epidermis of uncertain thickness, formed 
of one or more layers of colourless Gcllules closely 
packed together. This membrane is pier ced with 
aa the pores usually known by the name of stomata. 
ABSORPTION IN PLANTS. 
The doubts that have been entertained upon the 
existence of perforations in these stomata, M. 
Brongniart thinks he has removed, and that it is 
certain that in the centre of each stoma is an 
opening by which the outer air communicates 
with the parenchyma. This parenchyma is evi- 
dently the seat of respiration ; for it is the part 
that changes colour in exercising this function, 
which becomes green by the absorption of the 
carbon of the carbonic acid of the atmosphere, 
and which is discoloured again in darkness by 
the combination of the carbon of its juices with 
the oxygen of the air. This parenchyma differs 
entirely from that of other organs by the numer- 
ous irregular cavities that it contains, which com- 
municate with each other and the outer air by 
means of the openings of the stomata. It is into 
these cavities in the cavernous parenchyma of 
aerial leaves that the atmospheric air penetrates | 
when it is absorbed by the surface of the utricles 
of the parenchyma, that are distended with the | 
fluids which seem to nourish the plant. Accord- 
ing to M. Brongniart, aquatic leaves, if submerged, 
differ, in being completely destitute of epidermis. 
It is not alone stomata that they want, as has 
long been known, but the epidermis also, having 
no need of protection from rapid evaporation. 
There are none of the cavities that abound in 
the parenchyma of aerial leaves, but, on the con- 
trary, the cellules of the tissue are compactly 
fastened together without any interstice, and 
the air dissolved in the water can only act on 
their outer surface. For this reason the propor- 
tion borne by this surface to the whole mass of 
the leaf is unusually great; the leaves, from 
want of epidermis, dry up quickly when exposed 
to the air, and can only exist in water or a very 
humid atmosphere. 
that the epidermis is destined to protect aerial 
leaves against too rapid evaporation, and the | 
stomata or pores of this epidermis become neces- 
sary to maintain a communication between the 
atmosphere and the parenchyma. 
The existence of pores, even in the minute ori- 
fices called stomata, is questioned by some very 
distinguished botanists ; and the existence of sto- 
mata themselves is seldom detected in the epi- 
dermis of roots, bulbs, flowers, and fleshy fruits 
which require free exposure to the air, or at least 
a free infiltration of it, in order to their continu- 
ing healthy or arriving at maturity. Some recent 
experimenters and physiologists, however, have 
attempted to prove that gases may be inhaled 
and moisture imbibed, through either the vege- 
table or the animal membrane, without the in- 
tervention of any visible pores,—or that gases 
may be inhaled and moisture imbibed, by either 
an exceedingly powerful organic infiltration of 
the molecules or incomprehensibly small ultimate 
atoms of matter, or by the inward rush of a less 
dense to a more dense fluid, excited by electricity. 
But further explanation on this point will be 
given in the articles Enposmosr and AscENT oF 
Hence the author concludes | 
