THE COTTAGE GARDENER AND COUNTRY GENTLEMAN, October 18, 1859. 
41 
and regarding the upper surface of the leaf of the herb as being 
endowed with the greater absorbing power, owing to its low 
stature, and to the slow ascent of exhalations near the earth.— 
( Keith's Botanical Lexicon.) 
During the day leaves also absorb carbonic acid gas, which 
they decompose, retaining its carbon, and emitting the greatest 
part of the oxygen that enters into its composition. In the night 
this operation is in a certain measure reversed, a small quantity 
of oxygen being absorbed from the atmosphere, and a yet 
smaller proportion of carbonic acid emitted. 
It has occasionally been observed, however, that the bulk of 
oxygen given off by the leaf has not been precisely equal to that 
of the carbonic acid absorbed, and hence it is also fairly con¬ 
cluded that a portion of the oxygen of the carbonic acid which 
enters the leaf is retained, and made available in the production 
of the various substances which are formed in the vascular system 
of different plants. On the other hand, it is stated by Sprengel 
that, if compounds containing much oxygen be presented to the 
roots of plants, and thus introduced into the circulation, they 
are also decomposed, and the oxygen they contain in part or in 
whole given off by the leaves, so that, under certain circumstances, 
the bulk of the oxygen which escapes is actually greater than 
that of the carbonic acid which is absorbed by the leaves. Such 
is the case, for example, when the roots are moistened with 
water contained carbonic, sulphuric, or nitric acids. 
As a general rule, the quantity of carbonic acid given off 
during the night is far from being equal to that which is ab¬ 
sorbed during the day. Still it is obvious that a plant loses 
carbon precisely in proportion to the amount of this gas given 
off. Hence, when the days are longest, the plant will lose the 
least, and where the sun is brightest it will gain the fastest;— 
since, other things being equal, the decomposition of carbonic 
acid proceeds most, rapidly where the sky is the clearest, and the 
rays of the sun most powerful. It thus appears why in Northern 
regions, where spring, summer, and autumn are all comprised in 
one long day—vegetation should proceed with such rapidity. 
The decomposition of the carbonic acid goes on without inter¬ 
mission, the leaves have no night of rest; but Nature has kindly 
provided that, where the season of warmth is so fleeting, there 
should be no cessation to the necessary growth of food for man 
and beast.—( Johnston's Lectures on Agricultural Chemistry.) 
Carbonic acid gas in small proportions is essential to the 
existence of leaves, yet it only benefits them when present in 
quantities not exceeding one-twelfth of the bulk of the atmo¬ 
sphere in which they are vegetating; though one twenty-fifth is 
a still more favourable proportion ; and as hotbeds, heated by 
fermenting matters, rapidly have the air within their frames con¬ 
taminated to a much greater extent than the proportions above- 
named, thence partly arises the injury to the plants they contain 
from a too dong neglected ventilation. The leaves turn yellow 
from the excess of acid, which they are unable to digest, and 
which consequently effects that change of colour which also 
occurs in autumu, and which will be more fully considered when 
the decay of plants is detailed. 
It is the accumulation of carbonic acid and other gaseous 
matters, such as sulphurous acid and ammonia, which renders 
ventilation so essential to the health of plants in forcing-pits and 
hothouses. They cannot inhale air overloaded with these con¬ 
taminations without being speedily injured, and the proportions 
of those gases which rapidly cause disease, or even death, are 
much less than the gardener usually suspects; for if the sulphu¬ 
rous acid amounts to no more than one cubic foot in ten thou¬ 
sand of the air in a hothouse, it will destroy most of its inha- i 
bitants in two days. To avoid such destruction, for the comfort 
of visitors, and, above all, for the sake of the plant’s vigour, air 
should be admitted as freely as the temperature will permit. The 
foul warm air can be easily allowed to escape through ventilators 
in the most elevated parts of the roof, and fresh warm air can be 
as readily supplied through pipes made to enter near the flooring 
of the house after passing over hot water, or other source of heat. 
We are quite aware that Mr. Knight has stated that he paid 
little attention to ventilation, and that plants will be vigorous 
for a time in Wardian cases; but this does not prove that their 
Creator made a mistake when he placed vegetables in the open 
air. Plants confined in houses or other close structures may be 
made to grow in spite of such confinement; but all experience 
proves that other favourable circumstances, such as heat, light, 
and moisture, being equal, those plants are most vigorous and 
healthy which have the most liberal supply of air. 
Though an excess of carbonic aid gas is detrimental, yet its 
partial absence from the atmosphere is equally fatal to a plant’s 
leaves, for without it they wither and fall. It is not a matter of 
indifference, therefore, whether a greenhouse or hothouse be 
whitened with a solution of lime, which absorbs that gas from 
the air, a fortnight or only a day or two before plants are in¬ 
troduced or forcing commenced ; for it is the infliction of several 
trivial injuries to a plant that prevents its successful cultivation ; 
no one who is entitled to practise in the higher departments of 
his art ever makes such great blunders as at once to destroy the 
plants under his care. That fresh-limed walls do injure plants 
is beyond dispute, for the plants in a row of small pots next the 
back wall in a propagating-house which had been thus whitened 
only the day before, have been more than once observed to be 
the only plants that acquired a sickly hue, and shed nearly all 
their leaves. Eleshy-leaved plants would not be so liable to 
injury if obliged to be brought into a house fresh limed, for these 
require much less carbonic acid daily than thin-leaved plants. 
Eive plants of Cactus speciosissimus in the injured row just 
noticed were not apparently affected. Thin-leaved plants con¬ 
sume daily from five to ten times their own bulk of carbonic acid 
gas, whilst fleshy-leaved plants, such as the Cacti, Aloes, Agaves, 
and Mesembryanthemums, do not consume more than their own 
or double their own bulk of that gas. 
Other species of decomposition also, besides that of carbonic 
acid, go on in the leaf, or are there made manifest. Thus when 
plants grow in a soil containing much common salt (chloride of 
sodium) or other chlorides, Sprengel and Meyen observed them 
to evolve chlorine gas from their leaves. This takes place, how¬ 
ever, more during the night than during the day. Some plants 
also give off ammonia, while others (Cruciferse) emit from their 
leaves pure nitrogen gas (I)aubeny' s Three Lectures on Agri¬ 
culture, p. 59). This emission of nitrogen from the leaves is, 
according to Schultz, not an uncommon occurrence, and on a 
dark day may amount to nearly two-fifths of the entire bulk of 
the gas given off.—J. 
(To he continued.) 
HARDY FLOWERING HERBACEOUS PLANTS. 
(Continued from page 21.) 
ASTER—( Continued). 
Aster mutabilis (changeable). Stem smooth ; tranches twiggy ; 
leaves sub-stem-clasping, upper ones lanceolate-acuminate, 
entire, lower ones lanceolate, narrowed at the base, serrated ; 
involucre shorter than disk, lax. 2 ft. Purple. September. 
N. America. 
A. nemoralis (grove). Branches corymbose-fastigiate; branch- 
lets filiform, one-flowered; leaves linear-lanceolate, narrowing 
at base, nerveless, dotted, rather scabrous, stem-ones reflexed ; 
involucre lax, imbricated, half length of disk, leaflets pointed. 
1 ft. Lilac. August. N. America. 
A. NovAJ-ANGiliE (New England). Stem simple, erect, hispid ; 
flowers terminal, crowded; leaves lanceolate, stem-clasping, 
entire, eared at base ; involucre scales lax, lanceolate, coloured, 
longer than disk. 6 ft. Purple. September. N. America. 
A.- ruber (red-flowered). 6 ft. Red. July. North 
America. 
A. Novi Belgii (New York). Stem terete, smooth, paniculate ; 
branches sub-divided ; branchlets one-flowered, solitary ; leaves 
sub-stem-clasping, lanceolate, smooth, edge scabrous, lower 
ones serrated; invohicre loosely imbricated, leaflets linear- 
lanceolate. 4 ft. Purplish-blue. September. N. America. 
A. nudielorus (naked-flowered). 1 ft. Purple. August. North 
America. 
A. paladosuS (marshy). Leaves linear, stem-clasping, erect, 
entire, very smooth, edge scabrous; peduncles nearly naked; 
involucre squarrose, with two leaves beneath. 3 ft. Blue. 
August. N. America. 
A. Pannonicus (Hungarian). Stem simple, smooth at the top, 
corymbose; leaves linear-lanceolate, entire, edge hispid; in¬ 
volucre scales lanceolate, blunt, equal. 2 ft. Violet. July. 
Hungary. 
A. patens (spreading). Stem branched, hairy ; branches spread¬ 
ing, long, few-flowered; leaves oblong-lanceolate, entire, cor- 
dately stem-clasping, downy on each side; involucre scales 
imbricated, lanceolate, rather spreading. 2 ft. Purple. Oct. 
N. America. 
A. peregrinuS (foreign). 1ft. Blue. July. N. America, 
