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TRANSACTIONS OF SECTION B. 669 
which no longer took any account of the decomposition of carbon dioxide by the 
leaves, but which derived the whole of the elements of the growing plant from a 
solution of the soil extract taken up by the roots. We may well say with Sachs, 
‘ Nothing can be conceived more deplorable than this theory of nutrition ; it would 
have been bad at the end of the seventeenth century, it is difficult to believe 
that it could have been published thirty years after De Saussure’s work.’ It 
is well known how by the cogency of his reasoning and the force of his 
genius Liebig successfully overthrew this heresy, and once more established 
the doctrine of carbon assimilation as taught by De Saussure; and the accurate 
work of Boussingault, who, whilst elaborating far more delicate analytical 
processes than were possessed by chemists in the early days of the century, 
still in the main used De Saussure’s methods, gave the final death-blow to 
the humus theory, at any rate in the crude form in which it was presented by its 
originators. No one since that time has questioned the fact that green plants owe 
the greater part of their carbon to atmospheric sources, and the accumulated 
experience of two succeeding generations of workers has added proof on proof of 
the correctness of this great generalisation. 
But whilst it cannot be doubted that green plants devoid of parasitic or sapro- 
phytic habit derive the principal part of their carbon from the air, is the experi- 
mental evidence at present so complete as to exclude all other sources of supply ? 
De Saussure himself certainly left the door open to such a possibility, and although 
Boussingault held a different view, we find Sachs as late as 1865 maintaining that 
it is not contrary to the generally accepted theory of assimilation to suppose that 
there are chlorophyllous plants which decompose carbon dioxide and at the same 
time absorb ready-formed organic substances whose carbon they utilise in the 
formation of new organs. 
Up to comparatively recently there was little or no experimental evidence to 
justify this supposition, for the early experiments of De Saussure on the influence 
of solutions of sugar, and of other organic substances, on growing plants, although 
yery suggestive, were not of a sufficiently precise nature to lead to any conclusions, 
and we must come down to within fifteen years of the present time for anything 
like a demonstration that the green organs of plants can, under favourable condi- 
tions, build up their tissue from already elaborated carbon compounds just as do 
the fungi and the non-chlorophyllous plants generally. 
The active centres of the decomposition of carbon-dioxide in green leaves are 
the chlorophyll corpuscles or chloroplastids, and the first visible indication of this 
decomposition is the formation within these chloroplastids of minute granules of 
starch whose presence can be shown by suitable micro-chemical means. I have 
elsewhere discussed the question of how far the appearance of this starch is depen- 
dent on the pre-existence of other carbohydrates of a simpler constitution, and also- 
the probability that the whole of the products of assimilation do not necessarily 
pass through the form of starch: this is a subject which need scarcely concern us 
at the present moment; it is sufficient to draw attention to the main fact that in 
an assimilating cell the chloroplastids, in the vast majority of cases, give rise to 
these minute starch granules, which once more disappear when the plant is placed 
in darkness, or when the air around it is deprived of carbon dioxide. Now in 
1883 Bohm made the interesting discovery that when green leaves are placed in 
the dark until the starch of their chloroplastids has completely disappeared, there 
is a reappearance of starch when the cut end of the leaf-stalk is immersed in a 
solution of cane-sugar and of dextrose, or when the leaf is brought directly in 
contact with solutions of these substances. He found, in fact, that the elements 
of the cell which, under ordinary circumstances, manufacture their materials for 
plant growth by the reduction of carbon dioxide under the influence of sunlight, 
can, under other conditions, supply their requirements from suitable ready-formed 
organic substances. These observations of Bohm were fully confirmed two years 
later by Schimper, and were subsequently much extended by A. Meyer and E. 
Laurent, who found that fructose, maltose, mannitol, dulcitol, and glycerol could 
also contribute directly to the nutrition of leaves. 
Bokorny, working with Spiroyyra immersed in dilute solutions, found that 
