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K.—BOTANY. 201 
however, did not convince the upholders of the humus theory; they, 
for the most part, were silenced by Liebig (1840) who, building on the 
foundation laid by de Saussure, argued that the only source of carbon in 
the green plant was that derived from the carbon dioxide of the air. 
The problem was settled beyond dispute—Liebig was deductive rather 
than inductive—by Boussingault who, in 1851, introduced the method 
of water culture and proved that the ordinary green plant could flourish 
in a soil bereft of organic matter and that its nitrogen came from the 
nitrates of the soil, not from the air. 
Dutrochet (1837) correlated carbon assimilation with chlorophyll and 
was the first to use the escape of bubbles of gas from a cut shoot submerged 
in water as an indication of the process, a method adopted and improved 
by Sachs and used by him in quantitative experiments. 
Draper (1844) was the pioneer in establishing the connection between 
carbon assimilation and the quality of light. He showed that the curve 
of assimilation was almost negligible in the lower red rays, quickly rose 
to a maximum in the yellow green and gradually declined towards the 
blue, which observations were extended by Pfeffer and by Engelmann. 
The significance of light intensity was not appreciated until recently. 
Von Mohl (1851) considered that a carbohydrate was formed from the 
carbon dioxide, but it was Sachs who by experiment proved that the 
starch grains in the leaf, first described by von Mohl, were produced from 
carbon dioxide, and invented that experiment of our elementary classes 
which shows that the leaves of a green plant placed in the dark lose their 
starch and regain it on re-exposure to light. Sachs considered that starch 
was the first product of carbon assimilation ; it was not until some years 
later (1885) that Meyer showed that there were sugar leaves as well as 
starch leaves and thus indicated that sugar preceded starch in the process. 
Turning to the pigments associated with photosynthesis; Grew was 
the first to suggest that chlorophyll, using the word in its loose sense, 
was made up of more than one pigment, but to Stokes (1864) is due the 
eredit of making the first real advance and thereby initiating along series 
of investigations. By the fractional extraction of crude chlorophyll by 
organic solvents, Stokes demonstrated the presence of two green and two 
yellow pigments. These he studied optically and made certain observations 
on their chemical properties. His work was continued by others, notably 
by Fremy, Tmiriazeff, Kraus, Konrad, Sorby and particularly by Borodin, 
who in 1883 confirmed the discovery of the two carotinoids. The methods 
of separation of the mixed pigments were in time refined and culminated 
in the work of Tswett (1906) who, by his chromatographic method, 
separated two chlorophylls, thus confirming Stokes’ observation, and 
five carotinoids, of which one was carotin and the others xanthophyll. 
Many years were to elapse before the chemical constitution of chlorophyll 
was to be elucidated, but Hoppe-Seyler was the first to make the 
significant discovery, by the isolation of phylloporphyrin from chlorophyll 
in 1879, that there is a chemical relationship between the green pigment 
_ of the plant and the haemoglobin of blood. 
Turning to the mechanism of carbon assimilation, the formaldehyde 
hypothesis was originated by Butlerow (1861) who, by the action of alkali 
on trioxymethylene—a condensation product of formaldehyde—obtained 
