476 
NATURE 
[SEPTEMBER 14, 1899 
for the process. It only wanted, in fact, the recognition of the 
vegetable nature of the alga which constituted his ‘green 
substance” to bring these observations into line with his 
previous work, and to complete a discovery which would have 
eclipsed in importance all the others with which Priestley’s name 
is associated, It was just this one step which he most provok- 
ingly failed to take. It is true that he examined the “‘green 
substance”? under the microscope, but owing to want of 
skill in the use of the instrument, and also to his defective eye- 
sight, he was unable to determine its true nature, and unfor- 
tunately adopted the view that it had merely a mechanical 
action in separating the oxygen from the water, and, to use his 
own words, that ‘it was only a circumstance preceding the 
spontaneous emission of the air from water.” He was, in fact, 
now inclined to regard the process as a purely chemical one, 
due to the direct action of light on the carbon dioxide dissolved 
in the water. 
But this was by no means Priestley’s final view, as shown by 
a further description of his experiments on plants set forth in 
the new edition of his works published in 1790, where he clearly 
recognised the error into which he had been led.1_ Mean- 
while the subject had been taken up by two other observers, 
Ingen: Housz and Senebier, and in order to thoroughly under- 
siand the respective shares which these men took in advancing 
our knowledge of the assimilatory process, it is necessary to 
consult, not only their books, but also the numerous scattered 
memoirs which appeared at intervals between the years 1779 
and 1800. 
To Ingen-Housz must unquestionably be awarded the merit 
of having experimentally demonstrated that the amelioration of 
the surrounding air by plants is not, as Priestley at first believed, 
due to vegetative action fev se, but is dependent on the access 
of light of a sufficient degree of intensity, and, moreover, that 
the power is confined to the green parts of the plants. At the 
same time, whilst recognising, as Priestley had done before him, 
that the combined action of plants and light on the air was a 
dephlogisticating process, he did not know, until after its 
demonstration by Senebier, that the particular form of phlo- 
gisticated air which was essential to plants was “‘ fixed air” or 
carbon dioxide. In fact, Ingen-Housz had but a slender know- 
ledge of the chemistry of his day, so much so indeed that he 
constantly confuses ‘‘ phlogisticated air”’ or nitrogen with ‘‘ fixed 
air,” and attributes the source of the evolved oxygen either to 
air imprisoned within the leaf, or, in the case of submerged 
plants, to a metamorphosis of the water itself. I must, how- 
ever, recall the fact that Ingen-Housz was the first to show 
that the green parts of plants in the dark, and the roots both 
in the light and in darkness, vitiate the air in the same way as 
animals do. On the strength of these experiments, he is gener- 
ally given credit for having first observed the true respiration 
of plants, but I cannot avoid the conclusion that, in the con- 
troversy which ensued on this point between Ingen-Housz 
and Senebier, the adverse criticisms of the latter were well- 
founded. Whilst not denying that plants in the dark have 
some mephitic influence on the air around them, Senebier 
maintained that the greater part of the observed effect was due 
to a fermentative action set up in the large bulk of leaves which 
Ingen-Housz employed. Certainly some of the results appear 
to be largely in excess of those we should now expect to obtain 
from respiratory processes only.” 
Senebier’s work falls between the years 1782 and 1800. The 
fact that he was an early convert to the new ideas and general- 
isations of Lavoisier gives his views on plant nutrition far 
greater precision than those of Priestley and Ingen-Housz. 
His experiments, for the most part well devised, proved 
1 The view which was taken by Priestley’s contemporaries of his position 
with regard to the discovery of the fundamental facts is well exemplified by 
the following remarks taken from a paper published by Ingen-Housz in 
1784 (Annales de Physigue, xxiv. 44). ‘C'est A M. Priestley seul que nous 
devons la grande découverte que les végétaux possédent le pouvoir de cor- 
riger l’air mauyais et d’ameliorer |'air commun: c'est lui quinous en a ouvert 
la porte. J'ai été assez constamment attaché & ce beau systéme, dans le 
temps que lui méme, par trop peu de prédilection pour ses propres opinions, 
paroissoit chanceler.” 
2 It is by.no means uncommon to find Ingen-Housz put forward as the 
discoverer of the fixation of carbon by plants from carbon dioxide. This 
claim is generally based on certain statements made in his essay on the 
““Food Plants and the Renovation of the Soil,” published in 1796 as an 
appendix to the outlines of the fifteenth chapter of the ‘‘ Proposed General 
Report from the Board of Agriculture.” All that is good and sound in this 
essay is taken from Senebier’s papers without any acknowledgment, but, in 
appropriating ideas which he evidently understands very imperfectly, he 
has built up a system of plant economy which is almost unintelligible. 
NO. 1559, VOL. 60] 
beyond all doubt that the oxygen disengaged from submerged) 
and isolated plants could not be derived from air contained in 
the leaf parenchyma, but that it depended on the pre- 
existence of carbon dioxide, and that its evolution was strictly 
proportional to the amount of carbon dioxide which the water 
contained. ; 
Although positive experimental proof was still wanting that 
aérial plants also derive their carbon from carbon dioxide, 
Senebier regarded this as extremely probable; but, taking 
into consideration the small amount of this gas present in 
the atmosphere, he concluded that it must reach the plant by 
the roots and leaves entirely ina state of solution in water. 
The work of Priestley, Senebier, and Ingen-Housz fortunately 
attracted the attention of a young chemist of high attainments, 
who, within a period of less than ten years, did more for the 
advancement of vegetable physiology than any single observer 
before or since his time. Théodore de Saussure, the second of 
that illustrious name, and the son of the famous explorer and 
natural philosopher, commenced his researches about the year 
1796, and in 1804 published his ‘‘ Recherches Chimiques sur la 
Végétation,” a modest little octavo volume of some 300 pages 
which must certainly take rank as one of the great classics of 
scientific literature, and one of the most remarkable books of the 
century. 
De Saussure was a past master in the art of experiment, and 
the methods which he devised for demonstrating the influence of 
water, air and soil on vegetation have been the models on 
which all such investigations have been conducted ever since. 
It is indeed very difficult, when reading this masterly essay, to 
bear in mind that it was not written fifty or sixty years later than 
the date on its title-page, so essentially modern are its modes of 
expression and reasoning, and so far is the author in advance of 
his contemporaries. It is to this work we must refer for the first 
experimental proof that plants derive at any rate the greater 
part of their carbon from the surrounding atmosphere. This 
was shown by De Saussure by a variety of quantitative experi- 
ments of a sufficient degree of accuracy to bring out the great 
leading facts. By making known mixtures of carbon dioxide 
and air, and submitting them to the action of plants in sunlight, 
he was able, not only to show that the gaseous carbon dioxide 
was decomposed and the carbon assimilated, but also that the 
volume of oxygen disengaged was approximately equal to that 
of the carbon dioxide decomposed. He also showed that 
plants growing in the open in moist sand, or in distilled 
water, and therefore under conditions in which they could not 
derive any carbon from other than atmospheric sources, not 
only materially increased in dry weight, but contained much more 
carbon at the close of the experiment than at the beginning, 
and had also fixed an appreciable amount of water in the 
process. That atmospheric carbon dioxide is not only bene- 
ficial to plants in sunlight, but is also essential to their very 
existence, De Saussure proved by introducing an absorbent 
of this gas into the vessel containing a plant or the branch 
of a tree rooted naturally in the soil. Under these conditions, 
the portions of the plant enclosed always died. He also 
ascertained by experiment the increase in dry weight of a 
sunflower plant during four months of natural growth ; and 
knowing approximately the amount of water transpired during 
that period, and the maximum amount of solids which this 
transpired water could possibly introduce into the plant, he 
calculated that these solids, and the carbon dioxide in solu- 
tion in the transpiration water, fell far short of accounting 
for the observed increase in the dry weight of the plant. 
This increase must, therefore, be mainly due to the fixation 
of atmospheric carbon dioxide and water. 
It is certainly a remarkable fact that the rigid experimental 
proofs which De Saussure brought forward in support of his 
views did not carry conviction to the minds of every one. 
His book, however, suffered the fate of many others which 
have appeared in advance of their time. It is true that De 
Saussure’s doctrines were always kept alive by the advanced 
physiologists of the French school, such as De Candolle and 
Dutrochet, but when Liebig first turned his attention to the 
subject he found the field in possession of the humus theory 
of Treviranus, a theory which no longer took any account of the 
decomposition of carbon dioxide by the leaves, but which de- 
1 Although clearly indicating that no change of volume occurred in the 
mixture of air and carbon dioxide so treated, his final analytical results 
show a small apparent evolution of nitrogen. This was due to the eudio- 
metric methods he employed, methods, it is true, far superior in point of 
accuracy to those of his predecessors, but still necessarily imperfect. 
