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VEGETABLE PHYSIOLOGY. 
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By ARTHUR HENFREY, Esq., F.L.S., Lecturer ox Botany at St. George's Hospital. 
THE FOOD OF PLANTS. 
JjTlHE next question we have to deal with is that of the food of plants, and here it hecomes necessaiy 
21 to take note of two distinct hranehes of the subject, to the elucidation of which, chemistry, our 
great guide in these investigations, contributes very unequally. The first part of the question refers 
to the elementary nature of the indispensable food of plants ; and chemical analysis, by telling us the 
composition of the perfect organisms, at once answers our inquiries in this direction. But the second 
question, In what form or mode of combination do these elements enter the vegetable structures ? is 
very imperfectly solved by analytical research according to the present methods, and it is chiefly by 
means of experiment that we are enabled to obtain a few imperfect data for reasoning out definite 
views on this head. 
The first question may be quickly disposed of. Destructive analysis of vegetable substances, resolv- 
ing them into the simplest elements into which chemistry can at present separate the materials of 
creation, shows that those elementary substances which are met with constantly in plants amount only 
to eighteen, viz., the universal elements oxygen, carbon, hydrogen, and nitrogen ; the bases of the 
earths calcium, aluminum, silicium, and magnesium ; the readily acidifiable bases sulphur, phospho- 
rus, chlorine, iodine, bromine, and fluorine ; the bases of the alkalies potassium and sodium ; and the 
metals iron and manganese. These elements are not all of equal importance in the vegetable kingdom, 
a portion only being universally present, while some of the rest, which only occasionally occur, are 
even capable of replacing one another. Thus the first three, oxygen, hydrogen, and carbon are 
universal, forming the elements of which cell-membrane, starch, and sugar — those matters always 
present, are composed ; then nitrogen enters into the composition of all the albumen and similar pro- 
ducts contained in the cells, with which sulphur and phosphorus also are intimately connected, though 
in far smaller proportion. These may all be regarded as necessary constituents ; but the bases of the 
alkalies and earths appear to be less absolutely essential, since not only may one take the place of 
another, but it is stated that in some cases ammonia may be substituted for them ; this, however, must 
be looked upon as a very exceptional case, and, as a general rule, alkalies and earths, and mostly par- 
ticular kinds of earths, are necessary parts of the food of plants. Chlorine is a necessary constituent 
for certain plants ; iodine and bromine are of less account, while silica, iron, and manganese are met 
with less frequently, or in less abundance than any of the others ; save in certain special cases, as in 
those plants which, like the Grasses and Horsetails, have a silicified epidermis. 
Oxygen is the only one of these elements which is absorbed in a pure state into vegetables, all 
the rest are taken up in some condition of combination, capable of being decomposed by the plants 
themselves. One of the most vehemently debated controversies in the chemistry of vegetation has 
arisen from the different opinions held by different observers as to the nature of these combinations : 
whether they were necessarily organic or inorganic. As in so mauy other instances, the truth appears 
to beiu the middle, and unprejudiced inquirers will be inclined to believe from the evidence now before 
us, that both inorganic and organic compounds are absorbed by plants. 
Common observation, as well as direct experiment, prove that the majority of plants obtain the 
greater part of their food from iuorganic sources ; since when generations succeed each other for a 
long period the mass of decayed organic matter left behind by them undergoes continual increase, 
showing that they produce more than they consume. Saussure's experiments prove that plants will 
increase their organic matter when grown in water in a closed vessel containing an atmosphere rich 
in carbonic acid ; and plants have been made to grow, and even to flower and bear seed in earths 
which had been deprived of all organic matter by being heated to redness. This point is so clear that 
it now scarcely needs demonstration. 
But it is by no means proved that inorganic substances are the sole food of plants ; that the 
organic substances contained in black mould are only used after being decomposed into their inorganic 
constituents ; nay, in reality it is not shown that all plants are capable of living upon inorganic food. 
In the latter case we have the parasites which live upon the juice of living plants, and in many eases 
upon the sap of some particular plant or group of plants. Some of these parasites are, it is true, 
very peculiar in colour and habit, bearing leaves of imperfect character, and behaving as it were as 
flowers and fruits towards the plants which support them ; but a sufficient number, like the Loran- 
thacese, resemble other plants perfectly. Again, many plants will live only in decaying vegetable or 
animal substances, and a large portion of them resemble the brown fleshy parasites in external aspect, 
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