44 SECTIONAL ADDRESSES. 
C,H ,0,N,Fe, the non-albuminoid partner of globin in hemoglobin, 
Thus, omitting many intermediate stages, the relationship between 
chlorophyll and hemoglobin may be sketched by the following 
diagram :— 
Chlorophyll Hemoglobin---_——---—- rs peal 
Chlorophyllin AXtioporphyrin Hemin 
Pyrrophyllin a Ne Hematoporphyrin 
| 
Atiophyllin Hemoporphyrin 
It must be remembered, however, that although recent years have 
witnessed great progress in elucidating the nature of chlorophyll and 
hemoglobin, the mechanism by which they act remains unrevealed. 
The famous assimilation hypothesis of von Baeyer, according to which 
it is formaldehyde which represents the connecting-link in the phyto- 
chemical synthesis of carbohydrate from carbon dioxide, was enunci- 
ated in 1870, and arose from Butlerow's preparation of methylenitan. 
In spite of numerous criticisms, some of which are quite recent, it 
remains unshaken. The line of such criticism has taken two direc- 
tions. On the one hand, H: A. Spoehr (1913), from experiments 
suggested by the fact that the morning acidity of plant juices diminishes 
or disappears on exposure to light, has shown that this change is 
photochemical only, and maybe independent of enzymes, the volatile 
products including formaldehyde. Emil Baur (1908, 1910 and 1913) 
has urged the claims of oxalic acid to be regarded as the first product 
of assimilation, and shows how. this may lead to the other plant-acids, 
glycollic, malic and citric, the first-named being a possible stepping- 
stone to the carbohydrates by resolution into formaldehyde (and formic 
acid), incidentally assuming towards malic and citric acids the relation- 
ship which glucose bears to starch. On the other hand, K. A. Hofmann 
and Schumpelt (1916), preceded by Bredig (1914), have attacked the 
hypothesis on the. ground of kinetics, and imagine an electrolytic 
resolution of water under the influence of light, which liberates oxygen 
and effects the reduction of carbon dioxide by hydrogen to formaldehyde 
through formic acid. 
All these arguments have been weighed by Willstiitter and Stoll 
(1917), who dismiss them on comparing the volume of carbon dioxide 
absorbed by leaves with the corresponding volume of oxygen liberated. 
They point out that this assimilatory quotient, CO,/O,, which should 
be unity in the case of formaldehyde, becomes 1.33, 2 and 4 in the case 
of glycollic, formic and oxalic acids respectively. Proceeding to deter- 
mine this quotient experimentally they found it to be unity, whether the 
temperature is 10° or 35°, whether the atmosphere is rich in carbon 
dioxide or free from oxygen, ‘and alike with ordinary foliage or cactus. 
Furthermore, they found (1917) that whilst. organic liquids holding 
chlorophyll in solution do not absorb more carbon dioxide than the 
