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Carbon Assimilation. 
of extraction and variations in their amounts. It must, of course, 
be understood that anyone wishing to obtain first-hand knowledge 
of these aspects of the subject must consult Willstatter’s book (1913) 
or his original papers referred to therein. Most of the information 
given in the succeeding sections of this chapter is due to Willstatter 
and his co-workers. It is impossible to offer any criticism of the 
methods or the results of Willstatter, and in many cases in the 
following we have simply had to quote him without comment. 
In view of the information given by Willstatter’s researches it 
becomes unnecessary and would only be confusing, to enter into 
any discussion of the different results obtained by other continental 
investigators of this subject such as Hoppe-Seyler (1879,1880,1881), 
Gautier (1879) and Stoldasa (1907, 1909, 1913). It is sufficient to 
indicate that their results now appear due to their imperfect 
methods of extraction. 
The difficulties of isolating chlorophyll, partly because it 
changes so easily to other substances, and is so soluble in many 
solvents, has had the result of producing a very voluminous 
literature, but not even the most elementary questions had been 
solved before Willstatter’s researches. Thus it was not known 
whether there was one chlorophyll substance or more than one 
and as recently as 1906, Etard claimed to have found in one plant 
a whole series of different chlorophyll pigments, and an unlimited 
number of chlorophylls from different plants. Also the elementary 
questions of analysis had not been solved. It was not even known 
which elements were contained in the chlorophyll molecule. 
Now, owing to Willstatter’s work, which is undoubtedly one of 
the most brilliant achievements of organic chemistry, our knowledge 
of the chemistry of chlorophyll is as complete as, or more complete 
than, that of any other plant substance. His researches have 
therefore cleared the way for a vast amount of plant physiological 
work of the greatest importance. It seems impossible that this 
unique work of Willstatter and his co-workers should not influence 
and stimulate work in plant physiology, and it is surprising how 
little this work has influenced plant physiological research so far. 
Thus, in some recent work (Ewart, 1915) where it is contended 
that Willstatter’s methods of extraction have been followed, it 
would have been more convincing if the author of the paper had 
stated what chemical tests he applied to test the purity of his 
extracted pigment. 
(To be continued). 
