Pp 
K.—BOTANY, 209 
which, as is well known, is that the rate of diffusion through the pore of a 
stomate is governed by the law of diameters. 
Turning to metabolism, this third period is remarkable for the clearer 
focussing of the circumstances attending metabolic activity and especially 
of the action and interaction of those factors which influence the rate 
of particular activities. Before 1905, these factors were considered 
separately and various optima were given for various functions. The 
_ insufficiency of this was soon appreciated by F. F. Blackman, and his 
_ application of Liebig’s law of the minimum to physiological processes led 
to his conception of limiting factors, the immediate result of which was a 
clearer understanding of the activity of the green plant and of the factors 
which determine it and led to a stricter quantitative procedure in plant 
physiology. Following this conception, there was a renewed activity in 
the investigation of metabolic processes especially carbon assimilation 
and much discussion centred around the shape of the inflexion of the 
curve representing such activities. The truth of the doctrine is generally 
admitted, but the observations of Boysen-Jensen, Harder, Lundegardt, 
Warburg and others indicate that a particular factor is only strictly 
limiting when it is very much weaker than the others, and then the 
inflexion of the curve will be abrupt. When, however, the intensity of two 
factors are about equal and are limiting, the curve will be logarithmic. 
It has been mentioned that Hoppe-Seyler established a chemical 
relationship between chlorophyll and hemoglobin. His work was carried 
much further by Marchlewski, Nencki and Zaleski (1896). Schunck and 
Marchlewski (1899), by their use of alkali and acid on chlorophyll, isolated 
a numberof decomposition productssuch as alkachlorophyll, phylloxanthin, 
phyllocyanin, phyllotaonin and phylloporphyrin. Although these sub- 
stances were not pure and although their work did not throw much light 
on the changes effected on the chlorophyll by their treatment, it paved 
the way for the many investigations of Willstatter. Willstatter and his 
fellow workers by improved methods of experiment, especially fractional 
separation, obtained a number of products the phytochlorins and the 
phytorhodins. The fact was established that magnesium is an essential 
constituent of chlorophyll and that the pigment is an ester of the alcohol 
phytol. These facts led to the great discovery of the existence of two 
chlorophylls, their composition, and, later, of the chemical composition 
of the associated pigments carotin and xanthophyll. A few years after, 
Willstatter and his collaborators completely elucidated the composition 
of many anthocyanins and crowned the earlier work of Overton, Molisch, 
Grafe, and others. Here, again, this was made possible by Willstatter’s 
genius for refinement of technique which reached its apex in his recent, 
1926, work on enzymes. 
Of the sequence of events in the elaboration of food stuffs, but little 
real progress was made, notwithstanding a plethora of theories and an 
abundance of test tube observations. The work of Brown and Morris 
(1893) on the chemistry and physiology of foliage leaves, in which they 
Tecognised sucrose as the sugar first formed in carbon assimilation, is 
outstanding and was the first of many investigations amongst which the 
work of Parkin, Davis, Daish and Sawyer, Gast, Kylin and Weevers may 
be mentioned. 
1931 ic 
