86 
Ingvar Jorgensen and Walter Stiles. 
particularly small quantities of plant acids, will cause considerable 
alteration in the pigments. These considerations apply equally to 
the colorimetric and spectroscopic methods of estimation hitherto 
employed. 1 
Willstatter himself has so far used only colorimetric methods, 
for it is of no value to make measurements of a high degree of 
accuracy before possible errors in the extraction and separation of 
the pigments have been eliminated. When this has been done 
Willstatter suggests that quantitative spectroscopic analysis may 
prove very useful as a further refinement in the quantitative 
estimation of the pigments. 
We shall give in some detail Willstatter’s latest methods for 
the quantitative estimation of the pigments, more particularly as 
Willstatter in his latest publications (1915 b, 1915 c) applies 
them to plant-physiological work. 
It is scarcely necessary to emphasise the extreme importance 
of obtaining reliable methods for the determination of the 
quantities of pigments in leaves. While Willstatter’s earlier work 
in this respect, which is published in his book (1913), was mainly 
done in order to test the validity of his methods, yet he made also 
some estimations to determine whether there was any regularity in 
the variations in the quantity of the pigments in leaves, these 
estimations yielded figures very suggestive with regard to the 
physiological function of the pigments. Now Willstatter has 
definitely taken up the plant physiological aspect of this question, 
but it must not be forgotten that his work in this regard has so far 
been very limited and undertaken from a purely chemical point of 
view. It remains for the plant physiologist and ecologist to take 
up the methods and apply them in their various departments of 
research. 
Of course the technique of these methods is not to be acquired 
without some trouble and practice, but their employment appears 
at present the only way to reliable results. 
It is not our intention to give an historical survey of the 
considerations which led Willstatter to the methods he ultimately 
adopted. They were developed for the purpose of comparing the 
1 For instance, in reference to a recent paper by Jacobson and March- 
lewski (1912) where the authors claim to have shown that climatic conditions 
play an important part in regard to the production of one or other of the 
chlorophyll components, Willstatter points out that some of the errors com¬ 
mitted by these workers were (1) only a fraction of the chlorophyll present 
was extracted, (2) an unknown portion of the extracted pigment was precipi¬ 
tated as phaeophytin, and (3) only a portion of the phaeophytin was isolated. 
