SEPTEMBER 14, 1899] 
NATURE 
477 
rived 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 Boussingauit, 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 saprophytic habit derive the principal part of their 
carbon from the air, is the experimental evidence at present so 
complete as to exclude all other sources of supply? De 
Saussure himself certainly left the door open to such a possi- 
bility, 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 experi- 
mental evidence to justify this supposition, for the early ex- 
periments of De Saussure on the influence of solutions of 
sugar, and of other organic substances, on growing plants, 
although very suggestive, were not of a sufficiently precise 
nature to lead to any conclusions, and we must come down 
to within fiftteen years of the present time for anything like 
a demonstration that the green organs of plants can, under 
favourable conditions, 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 dependent 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 assim- 
ilating 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 thedark until the starchof 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, manu- 
facture 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 Sfzvogyra immersed in dilute solu- 
tions, found that starch production in the chlorophyll bodies 
could be induced by a large number of organic substances, in- 
cluding, amongst many others, asparagin, citric, tartaric, and 
lactic acids, leucine, tyrosine, and peptone. 
1 By far the most interesting and important result of Bokorny is the proof 
he gives that formaldehyde is directly assimilable by Spirogyra. His early 
attempts to show this had been rendered abortive by the highly poisonous 
NO. 1559, VOL. 60] 
Very much more to the point are the experiments of Acton, 
made in 1889, and the still more recent work of J. Laurent and 
of Mazé. 
In his experiments on terrestrial plants, Acton, after depleting 
them of starch, immersed the cut branches or roots, as the case 
might be, in culture fluids containing certain organic substances, 
and took precautions to prevent any normal assimilation from 
taking place by depriving the airaround the plant of any trace of 
carbon dioxide. He was not able to show the direct nutritive in- 
fluence of so large a range of substances as Bokorny had done for 
Spirogyra, but his results leave no room for doubt that several 
of the carbohydrates, and even glycerine, can be absorbed by 
the roots, and can contribute to the nutrition of the green parts. 
Acton tried, amongst other substances, an ‘‘ extract of natural 
humus,” which was an aqueous solution of the extractives of a 
light soil which are soluble in dilute alcohol. This extract was 
found to be effective in producing a small quantity of starch in 
the leaves, and it evidently contained some substance or sub- 
stances directly assimilable by the plant. 
Apparently without knowing anything of this work of Acton, 
J. Laurent has recently made a series of experiments on the 
culture of the maize plant in mineral solutions containing sac- 
charose, glucose, or invert-sugar, and in this way has not only 
obtained, as Acton had done before him, evidence of the active 
formation of starch in the leaves, but has also found a very 
notable increase in the dry weight of the plant. Although as- 
similation of the carbohydrate may under these circumstances 
go on in darkness, Laurent found that the process was much 
enhanced when light had access tothe plant. Mazé, within the 
last few months, has obtained even more pronounced effects of 
this kind. 
When all these new facts are taken into consideration, I think 
they justify what I have already said, that we ought to demand 
more direct evidence than is at present available before we 
accept the view that the majority of chlorophyllous plants take 
in the whole of their carbon from the atmosphere. In the cycle 
of change which the organic matter of the soil is constantly 
undergoing under the influence of micro-organisms, it seems by 
no means improbable that intermediate substances may be 
formed which in some measure directly contribute to the nutri- 
tion of the higher plants, and we must also by no means lose 
sight of the possible effect, in the same direction, of the symbiotic 
union of certain fungi with the root extremities of many plants, 
the Mycorhizz, whose functions are still so imperfectly under- 
stood. Then, again, we must remember that we have another 
possible extra-atmospheric source of carbon dioxide in the 
transpiration water of the plant, which is derived from a soil 
whose gases may contain 5 per cent. or more of carbon dioxide. 
From the amount of water transpired in a given time, and an 
application of the law of partial pressures, it may be readily 
shown that the supply of carbon dioxide to the aérial organs of a 
plant from this source is by no means negligible. 
Before these problems can be attacked for a particular plant 
with any hope of success, it is clear that we must have some 
means of establishing an accurate debtor and creditor account as 
between the plant and the surrounding atmosphere, and this 
account must extend over a sufficiently long period, and allow 
of an accurate balance being struck with the amount of carbon 
found in the plant at the end of the experiment. 
Up to within a few years ago we had no means of even ap- 
proximately determining the actual rate at which the assimilatory 
process goes on in a plant other than that afforded by its increase 
in weight in a given time. Such experiments, necessarily ex- 
tending over weeks or months, can, at the best, only give us 
certain average results, and consequently afford no measure of 
the activity of assimilation under fixed conditions of insolation. 
In the year 1884, Sachs, who had for some time been at work on 
the formation of starch in leaves under the action of sunlight, 
found that the accumulation of freshly assimilated material in a 
leaf may, under favourable conditions, go on so rapidly as to 
give rise to a very appreciable increase of weight in the leaf 
lamina within the short space of a few hours. By observing at 
nature of this substance. The difficulty was surmounted by using a dilute 
solution of sodium oxymethylsulphonate, which ‘on warming with water 
splits up into formaldehyde and acid sodium sulphite. To prevent the un- 
favourable action of the acid sodium sulphite, dipotassium or disodium 
phosphate was added to the plant cultures. In such a solution, with rigid 
exclusion of carbon dioxide, Spirogyra majyuscula forms starch in its 
chlorophyll bodies, but the access of light appears to be necessary. 
The importance of this experiment is very great in connection with 
Baeyer’s well-known hypothesis that the first act of assimilation is the 
reduction of carbon dioxide and water to the state of formaldehyde. 
