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with sand, or in fact with any other kind of earth. Heath mould, so often recommended
in the cultivation of the finer kinds of Australian plants, is, in fact, notliing
else hut peat earth rendered fertile hy the admixture of sand.
1157. The importance o f mineral substances was not formerly generally known,
as, before Liebig promulgated his discoveries, it was supposed by most vegetable physiologists
that tho mineral substances wliich were occasionally found in plants in minute
({uantities, “ were accidental and unimportant.” I t is worthy of remark, however, that
Dr. Lindley appears to have been awai'c of the absolute necessity which plants had for
mineral substances in their food, before the fact was generally acloiowlcdged ; as, in his
Theory o f Horticulture, published before Liebig’s great work, ho obseiwes, “ that whore
mincriil substances occur abundantly in plants, they ai-c part and parcel of their nature,
just as much as iron and phosphate of lime arc of our own bodies ; and wc must no
more su].)pose that grasses can disjieuse with silica in thcir food, or marine [ilants with
cominon salt, than that wc ourselves could dispense with vegetable and animal food.”
(Theory o f Horticulture, p. 356.)
1158. The food o f plants consists of carbon, oxygen, hydrogen, nitrogen, and various
earthy and saline substances; and these they obtain from earth, air, and water. The
atmospheric air consists of oxygen, nitrogen, and a small proportion of carbonic acid gas.
I t also contains hydrogen in the form of aqueous vapour and ammonia. The soil contains
all the vai'ious kinds of earthy and mineral matters, and a proportion of organic m a tte rs;
and water not only supplies plants with hydrogen and oxygen, when it is decomposed,
but it serves as a medium for dissolving vai-ious salts and other substances, and enabling
the spongioles of the roots to absorb them.
1159. Carbon. As carbon constitutes nearly one half of cvei-y vegetable substance, it is,
of course, the most important element in the food of [ilants. As it is a solid substance
insoluble in water, it cannot be taken up by th e spongioles of plants in tbat fonn : it can
only be imbibed by the plants when it is in combination witli oxygen, or, in other words,
in the state in which it is found in the atmosphere.
1160. Atmospheric air consists principally of nitrogen and oxygen, but it also contains
a portion of carbonic acid gas, which is produced by the breathing of men and other
animals, &c. I t is well known that atmospheric air is essential to the support of animal life,
and that the lungs of animals require a constant supply of fresh oxygen, and reject
the cai-bonic acid it contains; which, if they arc forced again to inspire it, is dcstnictivc
to life. Thus, if a human being is shut up in a coniiiicd space, where he can obtain no
fresh air, but is forced to breathe again the air which has already passed through his lungs,
aud which he lias already carbonated, he will soon die of suffocation. In the like
manner, people ai'C killed by the fumes of charcoal, which soon generates a sufficient
quantity of carbonic acid gas to destroy life. I t is a beautiful example of the economy
of nature, that the same gas which is injui’ious to animals is useful to plants, and that
plants by absorbing tho carbonic acid gas which they find in the atmosphere, pm-ify it,
and render it wholesome for human beings.
1161. Oxijgen. The proportion of oxygen in plants is, gcneraliy speaking, less than
of the carbon, bnt it is equally essential to thcir existence, there being very few vegetable
substances which do not contain more or less of it. Plants always absorb by thcir roots
more oxygen tlnm they arc able to assimilate, taking it in combined with carbon or
carboiiic acid gas, and united to hydrogen as water ; hence a gi-owing healthy plant is
constantly exhaling this supci-fiuous oxygen.
1162. Hydrogen, though forming a much smaller proportion of the food of plants than
either carbon or oxygen, is equally neccssai-y to them, and it is obtained partly from the
decomposition of water, and partly in combination with nitrogen, in the form of
ammonia.
1163. Nitrogen is found in plants in very small proportions, and it is generally taken
up by them combined with hydrogen, or, in other words, as ammonia. “ In lact,” as
Liebig observes, “ science is at present ignorant of any compound of nitrogen except
ammonia, whieli is capable of yielding nitrogen to the wild plants on all jinrts of tlie
earth’s surface.” (Liebig's Chemistry, p. 214.) Nitrogen is, however, occasionally supplied
to [ilants ill another form, that of nitric acid, and, with the exception of these compounds
(nitric acid and ammonia), “ nitrogen exists only in the form of a gas which has
been recognised as one of the constituent parts of atmospheric air.” (Ibid.) It was long
supposed that plants possessed tho power of assimilating the nitrogen contained in the
atinosphcro, though chemists were unable to explain how this was effected. The
necessity for it, however, is now obviated by tho discovery of ammonia in the air, from
the remains of decayed animal matters being dispersed in the atmosphere.
1164. Water is an essential part of the food of plants, partly because it sen-cs as a
medium by which they can imbibe other substances; and partly because they can
decompose it, and assimilate its constituent pai-ts.
1165. The inorganic substances which are found in plants consist chiefly of cai-thy and
ibrm of ash® when L l p t a T l ? Z L iL " " * «
eonsKlcrcd ¡is exactly reprcscntiiiff the I'nnvo-n»;»! ? V i ? 7 ’ cannot bo
in burning, various I g L e a L S L l d S S
viously combined; bonce the ashes of L ? allabno bases were prc-
Tbo substances thus left wlicn a plairt'is biirnt aioLomréh T l «Ikalinc.
carbonated, being derived from the dcstructioi, L L ™ “ ft“ ®“ «
acids; salts of the samo substances and mao-ncsia w itb s ifo b ft?™^“? ?
cblorinc, & e „_ sib ea, and tho oxido7of f r L a? 1 L L ? » phosphor,e adds,
o f organic acids are destroyed by burning so are tbofe of ‘''.« ®“ '« sd ts
various q L m i t e i r d i L r o t ; L d
with some other s,distance «'«®« always iound m combination
obsei-ve.;
wateL” p ^ ® “ ft™ * "« “™ "M l <lissolved in
lui e,auicu cuitme ; but when the water cannot escane from the <ieil o,iri to«f
and unfcrtiIc^bocnifor*"'n7! matter, the soil becomes converted into bog earth,
ft“ ®«'l"«»‘>y ««“ Ot be taken up by tho
1109. Aff sods wluch abound in certain salts must be fertile ■ and this is tlieressnn fb»f
L 7 L ? t e L t e d V i ° , ‘" ; r ’" “ ? f t ' Tft"- — w i è n r è s t
L tl ? L ? “ '« Prodnctivcncss of tlio soil is generally very greatly mcrcased
as tl e rocky subsoil becomes gradually disintegrated, partly by m e e lm t “ a i T , , 7 R
i b o L T t l L r A L f i l e ‘'“ '-l“ ',’‘“, T ‘ft “ ftft"®‘“ ‘ ®"WY noovo it. Ihus, a smface sod which lias a granito subsoil is certain toof b®o« *f‘e® r‘tiol et hnes' sthoed
S T s t d t d i t s L n t t a ' f t " ft‘Fft**' ®"fr®ofr"'fr'*of m aterialadrémtago
from b S t LtaiLd L a y . “ ‘"'ft™’“ ' ‘“ ft ' ° “ ' “ ft ‘“ ft ®«**
1170. In some sàuations irrigation has an almost m,ngical effect in increa-sinn- the
c o n t v c f t o L ‘-il ta“ fo.of the grass kind require silicate of potash, which is g cuF illy
convoyed to tho so ilb y the imgation of meadows. » facnei.uiy
1171. “P lu yh o ric acid has been found in the ashes o f all plants hitherto examined nud
always in combination with alksdios or alkaline earths. By biirniiio- the seeds of wheat
rye immze, peas, beans, aud lentils, ashes are obtained qfflto f a t rém c m t t L ’
and eons,stmg entirely of phosphates, with tbe exceptio? of r e i V s Z l L m r t i L of
sulphates and of chlorides.” (Xieto/s CAmista/, p .'i21.) Hams
1Ì 7 'f t''’ f- «°"®“ “ «>'‘ “f «fr fr“ l capable of cultivation,
in rome' 7 J^ tin g u ish ed as hot or cold, and the distinction appears
e Z o * “ fr«fr 'fr» “ /« " • Sotoo ®ofr® are much more heated by the iZ s Z f
ddeZviZce ooft ah ecaa t, cool in diftcrcnt Ztim"®es ;f tfir.. "c".* )s ome c«o‘“ol« ft®m 'u c«h"f rf a®s«tfer®r ftrhroa"ng fortt hteortsh c sTZhiies
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